Give an album a folder on the tablet, through Android's folder picker

Android's only export destination was the library on the server
(ExportTarget::available), because writing to the device goes through
the Storage Access Framework and nothing did. An album's folder on the
tablet is now chosen in the system's tree picker — which has its own
"Create new folder" — and exports are written into it with
DocumentsContract.

The picker answers through onActivityResult, and the main activity is
NativeActivity, whose result is not ours. FolderPicker is a translucent
activity that only asks: it starts ACTION_OPEN_DOCUMENT_TREE, takes a
persistable grant (a folder is chosen once and exported to for months),
leaves the URI in a static, and finishes. Rust polls it from a Slint
timer — one static call, rather than a registered native method and a
thread to deliver on.

Two things the first build on the tablet got wrong, recorded where they
are fixed:

- Our classes must be loaded through Context.getClassLoader(). The
  class of what ndk_context holds is a framework class from the boot
  loader, which reports every class in the APK as not found.
- What ndk_context holds is the application context, not the activity,
  and starting an activity from it throws without FLAG_ACTIVITY_NEW_TASK.

Saf.write creates the document (or, under Overwrite, reopens the one of
that name with "wt" so a shorter file does not keep the old tail) and
returns the name the provider actually gave it, since SAF renames on a
collision by itself; the album records that name. A tree URI reads in
the sidebar as its folder ("Pictures/Web"), not as a content:// string.

apps/darkroom-android/android/java/paris/tourolle/darkroom/Saf.java

@@ -0,0 +1,114 @@
+package paris.tourolle.darkroom;
+
+import android.content.ContentResolver;
+import android.content.Context;
+import android.database.Cursor;
+import android.net.Uri;
+import android.provider.DocumentsContract;
+import android.util.Log;
+
+import java.io.IOException;
+import java.io.OutputStream;
+
+/**
+ * Writing an export into a folder the user granted through
+ * {@link FolderPicker} — the Storage Access Framework, which is the only way
+ * this app reaches a folder on the device (FR-PLAT-AND-1).
+ *
+ * <p>A tree URI is not a path: a child is found by listing the folder and
+ * matching its display name, and created through the provider, which may
+ * rename it on a collision. So the name that was actually written is handed
+ * back, and the album records that one.
+ *
+ * <p>Two static calls, strings and a byte array in, a string out, for the
+ * reason {@link Intents} gives: every call here would be a signature typed as
+ * a string on the Rust side, and the fewer of those the better.
+ */
+public final class Saf {
+    private static final String TAG = "DarkRoom";
+
+    private Saf() {
+    }
+
+    /** Whether {@code name} already exists in the folder. False on any error. */
+    public static boolean exists(Context context, String tree, String name) {
+        try {
+            return find(context.getContentResolver(), Uri.parse(tree), name) != null;
+        } catch (RuntimeException e) {
+            Log.w(TAG, "checking " + name + " in " + tree, e);
+            return false;
+        }
+    }
+
+    /**
+     * Write {@code bytes} as {@code name} in the folder, replacing a file of
+     * that name when {@code replace} is set.
+     *
+     * @return the name the file has in the folder — the provider may have
+     *         added " (1)" — or null on failure, with the reason in the log.
+     */
+    public static String write(Context context, String tree, String name, String mime,
+            byte[] bytes, boolean replace) {
+        ContentResolver resolver = context.getContentResolver();
+        Uri treeUri = Uri.parse(tree);
+        try {
+            Uri target = replace ? find(resolver, treeUri, name) : null;
+            if (target == null) {
+                Uri folder = DocumentsContract.buildDocumentUriUsingTree(treeUri,
+                        DocumentsContract.getTreeDocumentId(treeUri));
+                target = DocumentsContract.createDocument(resolver, folder, mime, name);
+            }
+            if (target == null) {
+                Log.w(TAG, "the folder refused to create " + name + " in " + tree);
+                return null;
+            }
+            // "wt": truncate. A replacement shorter than what it replaces
+            // must not keep the old file's tail.
+            try (OutputStream out = resolver.openOutputStream(target, "wt")) {
+                if (out == null) {
+                    Log.w(TAG, "no stream for " + target);
+                    return null;
+                }
+                out.write(bytes);
+            }
+            String written = displayName(resolver, target);
+            return written != null ? written : name;
+        } catch (IOException | RuntimeException e) {
+            Log.w(TAG, "writing " + name + " to " + tree, e);
+            return null;
+        }
+    }
+
+    /** The document for {@code name} directly in the tree's folder, or null. */
+    private static Uri find(ContentResolver resolver, Uri tree, String name) {
+        String folderId = DocumentsContract.getTreeDocumentId(tree);
+        Uri children = DocumentsContract.buildChildDocumentsUriUsingTree(tree, folderId);
+        String[] columns = {
+                DocumentsContract.Document.COLUMN_DOCUMENT_ID,
+                DocumentsContract.Document.COLUMN_DISPLAY_NAME,
+        };
+        try (Cursor c = resolver.query(children, columns, null, null, null)) {
+            if (c == null) {
+                return null;
+            }
+            while (c.moveToNext()) {
+                if (name.equals(c.getString(1))) {
+                    return DocumentsContract.buildDocumentUriUsingTree(tree, c.getString(0));
+                }
+            }
+        }
+        return null;
+    }
+
+    private static String displayName(ContentResolver resolver, Uri document) {
+        String[] columns = {DocumentsContract.Document.COLUMN_DISPLAY_NAME};
+        try (Cursor c = resolver.query(document, columns, null, null, null)) {
+            if (c != null && c.moveToFirst()) {
+                return c.getString(0);
+            }
+        } catch (RuntimeException e) {
+            Log.w(TAG, "reading the name of " + document, e);
+        }
+        return null;
+    }
+}