Remove MediaPlaylistController, add MediaPlaylistAgent

MediaPlaylistAgent is the abstract class an application needs to
derive from to pass an object to a MediaSession2 that will override
default playlist handling behaviors. It contains a set of notify*
methods to signal MediaSession2 that playlist-related state has
changed.

Bug: 64098437
Test: make update-api
Change-Id: Icb3c57ddc14eba276f49d4ba85f11adbeb3e0917