MediaSessionRecord: Clean up codes

This includes following changes
- Remove unused public methods.
- Change the public method signatures not to share internal members
  when it's unncessary.

This is the initial step towards handling media key events via Media2.

Bug: TBD
Test: Build and flash manually
Change-Id: I4140abf26a1ccbc215a9a80ae0ff7636ae5fd698
3 files changed