Refactor opening output interface between AF and AP.

Refactor interface between audio flinger and audio policy. Use device
descriptor instead of audio device type and address. DeviceDescriptor
can contain more information that may be useful when opening an output
stream.
In audio policy, if multiple audio devices are passed in when opening
an output stream, one device is extracted using the same strategy as
when selecting a device for volume application. When starting source,
audio policy manager will call audio flinger to create audio patch. In
that case, multiple devices will still be patched to the output stream.

Test: atest AudioTrackTest AudioPlaybackCaptureTest
Test: atest audiopolicy_tests, audio smoke test
Change-Id: I46dc2da621614f8716c6c73cf807fe7d2d672cda