playback: apply the item's gain in one tested step

Review follow-ups.

- `ReplayGainController.applyItem` owns reading the gain off a
  `MediaItem`, so the transition, the controller and the extras read are
  exercised together rather than only in isolation.
- drop the leftover `setTrack(null)` on a missing active source: now that
  the transition sets the gain from the item, that line only raced it and
  could reset a playing item to unity.
- tests for a zero gain surviving the extras round trip (zero is a real
  gain, not an absent one), for the latest item's gain being the one left
  applied, and for a missing item applying unity.

app/src/main/kotlin/com/subtracks/playback/Media3PlayerHandle.kt

@@ -266,6 +266,10 @@ private fun Bundle.putGain(
 
 private fun Bundle.gain(key: String): Float? = if (containsKey(key)) getFloat(key) else null
 
+internal fun ReplayGainController.applyItem(mediaItem: MediaItem?) {
+    setTrack(mediaItem.replayGain())
+}
+
 internal fun queueMediaItem(
     item: QueueItem,
     local: String?,

app/src/main/kotlin/com/subtracks/playback/PlaybackService.kt

@@ -140,7 +140,7 @@ class PlaybackService : MediaSessionService() {
                     reason: Int,
                 ) {
                     observeCurrentSong(mediaItem?.mediaId)
-                    replayGain.setTrack(mediaItem?.replayGain())
+                    replayGain.applyItem(mediaItem)
                 }
             },
         )
@@ -187,11 +187,7 @@ class PlaybackService : MediaSessionService() {
         session?.setMediaButtonPreferences(listOf(starButton(false)))
         songJob =
             scope.launch {
-                val sourceId = sourceRepository?.activeSourceIdOnce()
-                if (sourceId == null) {
-                    replayGain.setTrack(null)
-                    return@launch
-                }
+                val sourceId = sourceRepository?.activeSourceIdOnce() ?: return@launch
                 library.song(sourceId, songId).collect { song ->
                     if (currentSongId != songId) return@collect
                     starred = song?.starred != null