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/AndroidManifest.xml

@@ -158,6 +158,18 @@
             android:theme="@style/ManualTheme"
             android:configChanges="orientation|keyboardHidden|screenSize|screenLayout|uiMode" />
 
+        <!-- FR-EXP-10: the system's folder picker, for an album's folder on
+             this device. NativeActivity's onActivityResult is not ours, so
+             this activity exists only to ask and hand the answer back (see
+             FolderPicker.java). Translucent and without a title so nothing
+             of it shows but the system chooser; not exported, and started by
+             class name from dr_ui::saf. -->
+        <activity
+            android:name="paris.tourolle.darkroom.FolderPicker"
+            android:exported="false"
+            android:theme="@android:style/Theme.Translucent.NoTitleBar"
+            android:configChanges="orientation|keyboardHidden|screenSize|screenLayout|uiMode" />
+
         <!-- FR-PLAT-AND-6, outbound. Android has refused file:// URIs
              between apps since API 24 — handing one out raises
              FileUriExposedException in *this* process — so an exported JPEG