README updates for session and media2

PiperOrigin-RevId: 402290115

extensions/media2/README.md

-# ExoPlayer Media2 extension
+# Media2 module
 
-The Media2 extension provides builders for [SessionPlayer][] and [MediaSession.SessionCallback][] in
-the [Media2 library][].
+The Media2 module provides builders for [SessionPlayer][] and
+[MediaSession.SessionCallback][] in the [Media2 library][].
 
 Compared to [MediaSessionConnector][] that uses [MediaSessionCompat][], this provides finer grained
 control for incoming calls, so you can selectively allow/reject commands per controller.

extensions/mediasession/README.md

-# ExoPlayer MediaSession extension
+# MediaSession module
 
-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.
+The MediaSession module mediates between a `Player` 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