Clean up Session2Record in MCS

This CL cleans up how MediaCommunicationService
manages Session2Record instances.

It removes unnecessary codes and refactor locks.
It also adds/clean up tests for MCM.SessionCallback.

Bug: 183289655
Bug: 182907935
Test: atest MediaCommunicationManagerTest 10 times
Change-Id: Iff4643c7dc207ea14c752bdfb815598dc34f2368