audio: Allow going to 'IDLE' for synchronous drain

For small buffers, the driver can perform draining
synhronously, returning control to the HAL only after
the buffer is empty. This makes going through
the 'DRAINING' state artificial. Thus, we allow going
to the 'IDLE' state directly.

In order to make sure that VTS handles both transitions:
to 'DRAINING' and to 'IDLE', correctly, add an "AOSP as
vendor" parameter "aosp.forceSynchronousDrain" to induce
this behavior in the default implementation.

Bug: 262402957
Test: atest VtsHalAudioCoreTargetTest
Change-Id: Ic8eaee53cb4596afb5317b4b905e004af3f112aa