Merge changes Iec1011a9,I64249876,Icb348a4a,Ibd46b7de,Ied45a5d7 into main

* changes:
  AudioFlinger: Do not dereference a nullptr for a reference
  AudioFlinger: Extract inner Thread classes
  AudioFlinger: Add MmapThread interfaces
  AudioFlinger: Add more Thread interfaces
  AudioFlinger: Add ThreadBase and PlaybackThread interfaces