[Media ML] Let MCS manage MediaSession2

This CL adds
 - MediaCommunicationManager#notifySession2Created
 - MediaCommunicationManager#getSession2Tokens
, which replaces the same methods in MediaSessionManager

to let MediacommunicationService manage MediaSession2.

MediaSessionService gets notified of created MediaSession2 instances
by adding a callback to MCM.

Bug: 180417011
Test: atest MediaSessionManagerTest MediaCommunicationManagerTest
Change-Id: Ia5ffdcd15573d1223ca520cfa8eca3b976874118