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 easiest way to use the extension is to add it as a gradle dependency:
implementation 'com.google.android.exoplayer:extension-mediasession:2.X.X'
where 2.X.X is the version, which must match the version of the ExoPlayer library being used.
Alternatively, you can clone the ExoPlayer repository and depend on the module locally. Instructions for doing this can be found in ExoPlayer's top level README.
com.google.android.exoplayer2.ext.mediasession.* belong to this module.