android: the downloads say so in the shade

A Save still running with the app in the background now posts a progress
notice — how many files, the one it is on, a bar, Stop all and Open — and
takes it down when the queue empties. It sits on its own low-importance
channel so it can be silenced without silencing the player's card, and
rides its own dataSync foreground service rather than the media one: a
mediaPlayback service stops itself when the bar empties, a download is not
playback, and nothing else was keeping the process alive for a Save with
nothing playing — which is how the first real Save died, four tracks in.
Stop all goes through the same cancel(id) a row's tap takes, and Transfers
grew the matching button beside Clear finished. The walk pushes two rows
straight into the transfer store and reads the real notification back out
of `dumpsys notification`, without fetching anything.

crates/tauri-plugin-transfer-notice/build.rs

@@ -0,0 +1,16 @@
+// `update` and `clear` are ours. registerListener / removeListener are the
+// mobile plugin runtime's own commands, the ones `addPluginListener`
+// invokes to hear the `action` event — commands like any other to the ACL,
+// and without them here the listener is refused and the notice's buttons go
+// nowhere. The media-session plugin carries the same scar.
+const COMMANDS: &[&str] = &["update", "clear", "registerListener", "removeListener"];
+
+fn main() {
+    // Registers the command permissions (permissions/autogenerated) and, on
+    // Android, hands the Gradle module under android/ to tauri-build, which
+    // includes it in the app project (tauri.settings.gradle) — but only with
+    // TAURI_ANDROID_PROJECT_PATH set, see ANDROID.md.
+    tauri_plugin::Builder::new(COMMANDS)
+        .android_path("android")
+        .build();
+}