MediaSession2: Clean up TODOs under frameworks/base

This change includes
  - Addressed trivial TODOs (takes less than 1min)
  - Renamed playlist controller to playlist agent
  - Renamed mplc to playlistAgent
  - Renamed mpb to player
  - Assigned buganizer issues for TODOs

Bug: 74361045
Test: Run MediaComponentsTest
Change-Id: I45e97081b5a3f24988c3daadd007edcb51be5288