docs(android): stop describing Picture-in-Picture, and say what PLAN.md has not done

Picture-in-Picture was removed from the Android client, but B40 still
read as a live fix, B42 stayed open on a `pending` PiP case that no
longer exists, and two code comments still named it. PLAN.md marked
every recommendation done; its status now lists the ones a source
review found short.

android/app/src/main/java/io/narl/protonstream/playback/NativeMpvHost.kt

@@ -245,7 +245,7 @@ class NativeMpvHost private constructor(private val nativeHandle: Long) : Libmpv
      */
     fun beginTransition() { mutableAdvancing.value = true }
 
-    /** Names the open episode for the media session, its notification and PiP. */
+    /** Names the open episode for the media session and its notification. */
     fun publish(playing: NowPlaying?) { mutableNowPlaying.value = playing }
 
     fun setPaused(paused: Boolean) {