Accommodate the move of AudioPlaybackRate to system/audio.h

AudioPlaybackRate is now defined in system/audio.h
as 'struct audio_playback_rate_t' (in global namespace).
Similar for the related enums.

To accommodate the existing code, introduce AudioPlaybackRate
as a typedef in the 'android' namespace.

Update code that was forward declaring AudioPlaybackRate
as it's not a struct anymore.

Bug: 133526565
Test: m
Change-Id: Iba20f1de1e8779cd78ac4573f2082416f2b428e2