audio: Generalize stream implementations

This allows for more code reuse and composability when
implementing streams for a particular audio "backend."

The existing "stub" code has been moved to StreamStub* files.

Bug: 264712385
Test: atest VtsHalAudioCoreTargetTest
Change-Id: I97fd41f87eb6d01e1d57f0d70a86d3b2b3555837