Clean up terminology for MediaSession extension

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=164705750

extensions/mediasession/README.md

@@ -2,10 +2,10 @@
 
 ## Description ##
 
-The MediaSession extension mediates between an ExoPlayer instance and a
-[MediaSession][]. It automatically retrieves and implements playback actions
-and syncs the player state with the state of the media session. The behaviour
-can be extended to support other playback and custom actions.
+The MediaSession extension mediates between a Player (or ExoPlayer) instance
+and a [MediaSession][]. It automatically retrieves and implements playback
+actions and syncs the player state with the state of the media session. The
+behaviour can be extended to support other playback and custom actions.
 
 [MediaSession]: https://developer.android.com/reference/android/support/v4/media/session/MediaSessionCompat.html
 

extensions/mediasession/src/main/java/com/google/android/exoplayer2/ext/mediasession/MediaSessionConnector.java

@@ -47,7 +47,7 @@ import java.util.Map;
  * Connects a {@link MediaSessionCompat} to a {@link Player}.
  * <p>
  * The connector listens for actions sent by the media session's controller and implements these
- * actions by calling appropriate ExoPlayer methods. The playback state of the media session is
+ * actions by calling appropriate player methods. The playback state of the media session is
  * automatically synced with the player. The connector can also be optionally extended by providing
  * various collaborators:
  * <ul>