Send media controller calls to background thread

This CL makes sure we don't create playback state action button on main
thread.

Flag: EXEMPT BUGFIX
Bug: 345304469
Bug: 364610391
Test: atest MediaDataProcessorTest
Test: atest LegacyMediaDataManagerImplTest
Change-Id: Ia0a4fabede4484fab91fb48911acb89660349780