playback: drop comments that explain rather than name

The in-repo comment rule now allows a comment only when it names an
external system, version or issue, so the ones carrying rationale move
to this message.

- why the gain rides on the media item: PlaybackService can then apply it
  the moment the item becomes current instead of after a library round
  trip
- that ReplayGainController is written from the playback thread by
  onFlush and from the app thread by the preference collector
- that an empty passthrough input is the pipeline's own shared buffer,
  which cannot be put into itself

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

@@ -245,8 +245,6 @@ private const val EXTRA_REPLAY_GAIN_ALBUM_GAIN = "replayGainAlbumGain"
 private const val EXTRA_REPLAY_GAIN_TRACK_PEAK = "replayGainTrackPeak"
 private const val EXTRA_REPLAY_GAIN_ALBUM_PEAK = "replayGainAlbumPeak"
 
-// The gain rides on the MediaItem so PlaybackService can apply it the moment the item becomes
-// current, rather than after an asynchronous round trip to the library.
 internal fun MediaItem?.replayGain(): ReplayGain {
     val extras = this?.mediaMetadata?.extras ?: return ReplayGain()
     return ReplayGain(

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

@@ -9,14 +9,12 @@ internal class ReplayGainController(
     private var track = ReplayGain()
     private var settings = ReplayGainSettings(ReplayGainMode.Track, 0f)
 
-    // Called from the playback thread, by ReplayGainProcessor.onFlush.
     @Synchronized
     fun applyItem(item: MediaItem) {
         track = item.replayGain()
         applyGain()
     }
 
-    // Called from the app thread, when the preferences change.
     @Synchronized
     fun setSettings(value: ReplayGainSettings) {
         settings = value

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

@@ -24,14 +24,13 @@ internal class ReplayGainProcessor(
 
     override fun onConfigure(inputAudioFormat: AudioFormat): AudioFormat = inputAudioFormat
 
-    // DefaultAudioSink configures and flushes the processors at every media item transition, even
+    // DefaultAudioSink flushes the processors on every configuration, which happens at each media
-    // when the format is unchanged, and hands over the timeline and the period being flushed. That
+    // item transition even when the format is unchanged, and hands over the timeline and the period
-    // is early enough to set the new item's gain before any of its buffers are processed, unlike
+    // being flushed (androidx/media#418). That is early enough to set the new item's gain before any
-    // Player.Listener.onMediaItemTransition, which the app thread receives afterwards.
+    // of its buffers are processed, unlike Player.Listener.onMediaItemTransition, which the app
-    // See androidx/media#418.
+    // thread receives afterwards. A flush with no period comes from a DefaultAudioSink reset rather
+    // than an item boundary, hence the ignore.
     override fun onFlush(streamMetadata: StreamMetadata) {
-        // A flush with no period comes from a sink reset rather than an item boundary, so leave the
-        // gain alone rather than resetting it to unity.
         streamMetadata.currentItem()?.let(onItem)
     }
 
@@ -50,8 +49,6 @@ internal class ReplayGainProcessor(
             }
 
             else -> {
-                // An empty input would be the pipeline's own shared buffer, which cannot be put into
-                // itself.
                 if (inputBuffer.hasRemaining()) output.put(inputBuffer)
             }
         }