Import from an SD card or card reader on Android

Import was switched off on Android: `imports_supported` was true only for
`target_os = "linux"`, and its comment said Android has no path to read a
card by and nowhere to write the copies. Neither holds. With "all files
access" (MANAGE_EXTERNAL_STORAGE, API 30) an app reads the root of an SD
card or a USB card reader by path, `/storage/9C33-6BBD`, and the importer
only ever writes into its own staging directory, which is a plain
directory on Android too. So the engine runs unchanged; what was missing
was finding the card and the permission.

- The manifest declares MANAGE_EXTERNAL_STORAGE, and
  READ_EXTERNAL_STORAGE up to API 29 with requestLegacyExternalStorage,
  which is the same access on 28 and 29.
- Cards.java lists the mounted non-primary volumes through
  StorageManager and opens the system "All files access" page for this
  app. dr_ui::cards is the JNI bridge, through saf's helpers.
- The import page on Android asks for the permission with an "Allow
  access" button until it has it, rather than showing an empty list that
  reads as "no card", and watches for the grant so the list fills in when
  the user comes back from settings.

Google Play restricts this permission to file managers and the like;
DarkRoom is sideloaded, so that does not apply.

ui/dr-ui/src/saf.rs

@@ -26,7 +26,7 @@ use jni::strings::JNIStr;
 /// Run `body` with the application context ndk_context holds, on whatever
 /// thread this is. It is a `Context`, not the activity — see
 /// `FolderPicker.start` for what that changes.
-fn call<T>(
+pub(crate) fn call<T>(
     what: &str,
     body: impl FnOnce(&mut jni::Env, &JObject) -> jni::errors::Result<T>,
 ) -> Result<T, String> {
@@ -62,7 +62,7 @@ fn call<T>(
 /// or the application context behind it) the boot loader defined, and the
 /// boot loader has never heard of anything in this APK. Loading through it fails with "class not found",
 /// which is what the first build on the tablet did.
-fn class<'local>(
+pub(crate) fn class<'local>(
     env: &mut jni::Env<'local>,
     activity: &JObject,
     name: &JNIStr,
@@ -80,7 +80,7 @@ fn class<'local>(
 }
 
 /// A Java string result, or `None` for null.
-fn text(env: &mut jni::Env, value: JObject) -> jni::errors::Result<Option<String>> {
+pub(crate) fn text(env: &mut jni::Env, value: JObject) -> jni::errors::Result<Option<String>> {
     if value.is_null() {
         return Ok(None);
     }