MediaPlaylistAgent: Add getMediaItem()

Provide a way for a custom MediaPlaylistAgent to tell which
MediaItem2 maps to the DataSourceDesc from the
MediaPlayerBase#PlayerEventCallback.

Bug: 74316764, Bug: 75500592
Test: Run CTS with MediaComponents/runcts.sh
Change-Id: I45c9893bf74b2635f31949be537885a79860cfb3