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: I44b4c7b543ac513914632e3ee235d3151840d3e0