playback: introduce foreground-safe restores

- Allow DeferredPlayback.RestoreState to force-start playback
- Allow DeferredPlayback.RestoreState to specify a fallback action
guaranteed to succeed

app/src/main/java/org/oxycblt/auxio/playback/service/ExoPlaybackStateHolder.kt

@@ -163,10 +163,18 @@ class ExoPlaybackStateHolder(
             is DeferredPlayback.RestoreState -> {
                 logD("Restoring playback state")
                 restoreScope.launch {
-                    persistenceRepository.readState()?.let {
+                    val state = persistenceRepository.readState()
+                    if (state != null) {
                         // Apply the saved state on the main thread to prevent code expecting
                         // state updates on the main thread from crashing.
-                        withContext(Dispatchers.Main) { playbackManager.applySavedState(it, false) }
+                        withContext(Dispatchers.Main) {
+                            playbackManager.applySavedState(state, false)
+                            if (action.play) {
+                                playbackManager.playing(true)
+                            }
+                        }
+                    } else if (action.fallback != null) {
+                        playbackManager.playDeferred(action.fallback)
                     }
                 }
             }

app/src/main/java/org/oxycblt/auxio/playback/state/PlaybackStateHolder.kt

@@ -276,7 +276,7 @@ data class QueueChange(val type: Type, val instructions: UpdateInstructions) {
 /** Possible long-running background tasks handled by the background playback task. */
 sealed interface DeferredPlayback {
     /** Restore the previously saved playback state. */
-    data object RestoreState : DeferredPlayback
+    data class RestoreState(val play: Boolean, val fallback: DeferredPlayback? = null) : DeferredPlayback
 
     /**
      * Start shuffled playback of the entire music library. Analogous to the "Shuffle All" shortcut.