audioflinger: new HW A/V sync ID allocation flow

The HW A/V sync ID is now allocated by the audio HAL before the
output stream is created by a call to global get_parameters() with
key AUDIO_PARAMETER_HW_AV_SYNC.

When the AudioTrack is created, the HW A/V sync ID is communicated
to the output stream by stream set_parameters() with key
AUDIO_PARAMETER_STREAM_HW_AV_SYNC.

Bug: 17112525.
Change-Id: Ia8bc6f3bf9f358aa89f3f56ac554e893a19811ad