audio policy: refactor HW audio sources

Refactor management of hardware audio sources to make use of
ClientDescriptor class:
- HW sources are now tracked by port ID and not audio patch handle.
- startAudioSource() and stopAudioSource() APIs are updated to use
audio_port_handle_t as source identifier.
- AudioSourceDescriptor class is deleted and replaced by
SourceClientDescriptor class deriving from TrackClientDescriptor

Test: make.
Change-Id: Ie418b566519a591f036b538a77319f8e30aa99a8