AudioFlinger: Add ThreadBase and PlaybackThread interfaces

ThreadBase::standby() check renamed ThreadBase::inStandby() to avoid future
confusion with an active method.

Test: atest audiorecord_tests audiotrack_tests audiorouting_tests trackplayerbase_tests audiosystem_tests
Test: atest AudioTrackTest AudioRecordTest
Test: YouTube Camera
Bug: 288339104
Bug: 289233517
Change-Id: Ied45a5d762b5a53c9d5dcd88d26efaa8b058836d