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()
+                val sourceId = sourceRepository?.activeSourceIdOnce() ?: return@launch
-                if (sourceId == null) {
-                    replayGain.setTrack(null)
-                    return@launch
-                }
                 library.song(sourceId, songId).collect { song ->
                     if (currentSongId != songId) return@collect
                     starred = song?.starred != null

app/src/test/kotlin/com/subtracks/playback/MediaItemTest.kt

@@ -76,4 +76,17 @@ class MediaItemTest {
 
         assertEquals(ReplayGain(), item.replayGain())
     }
+
+    @Test
+    fun aZeroGainIsNotMistakenForNoGain() {
+        val item =
+            queueMediaItem(
+                song.copy(replayGain = ReplayGain(trackGain = 0f, trackPeak = 0.5f)),
+                local = null,
+                stream = "http://server/stream?s=s1",
+                transcode = false,
+            )
+
+        assertEquals(ReplayGain(trackGain = 0f, trackPeak = 0.5f), item.replayGain())
+    }
 }

app/src/test/kotlin/com/subtracks/playback/ReplayGainControllerTest.kt

@@ -1,9 +1,12 @@
 package com.subtracks.playback
 
+import androidx.test.ext.junit.runners.AndroidJUnit4
 import com.subtracks.data.prefs.ReplayGainMode
 import org.junit.Assert.assertEquals
 import org.junit.Test
+import org.junit.runner.RunWith
 
+@RunWith(AndroidJUnit4::class)
 class ReplayGainControllerTest {
     private val applied = mutableListOf<Float>()
     private val controller = ReplayGainController { applied += it }
@@ -43,6 +46,52 @@ class ReplayGainControllerTest {
         assertEquals(1f, applied.last(), TOLERANCE)
     }
 
+    @Test
+    fun theLatestItemLeavesItsGainApplied() {
+        controller.setSettings(ReplayGainSettings(ReplayGainMode.Track, 0f))
+
+        controller.applyItem(item("a", trackGain = 6.0206f))
+        controller.applyItem(item("b", trackGain = -6.0206f))
+
+        assertEquals(0.5f, applied.last(), TOLERANCE)
+    }
+
+    @Test
+    fun anItemWithoutGainPlaysAtUnity() {
+        controller.setSettings(ReplayGainSettings(ReplayGainMode.Track, 0f))
+
+        controller.applyItem(item("a", trackGain = null))
+
+        assertEquals(1f, applied.last(), TOLERANCE)
+    }
+
+    @Test
+    fun noItemPlaysAtUnity() {
+        controller.setSettings(ReplayGainSettings(ReplayGainMode.Track, 0f))
+        controller.applyItem(item("a", trackGain = 6.0206f))
+
+        controller.applyItem(null)
+
+        assertEquals(1f, applied.last(), TOLERANCE)
+    }
+
+    private fun item(
+        id: String,
+        trackGain: Float?,
+    ) = queueMediaItem(
+        QueueItem(
+            id = id,
+            title = id,
+            artist = null,
+            album = null,
+            coverArtId = null,
+            replayGain = ReplayGain(trackGain = trackGain),
+        ),
+        local = null,
+        stream = "http://server/$id",
+        transcode = false,
+    )
+
     private companion object {
         const val TOLERANCE = 0.001f
     }