Add unique audio port IDs to AudioTrack and AudioRecord

This will allow to track activity at the track level instead of
at audio session level as only possible with current implementation.

AudioTracks and AudioRecords will receive a unique audio port ID the
first time they register to audio policy with
getOutputForAttr()/getInputForAttr() and keep this ID for their
lifetime.

This CL is the first partial change and just updates the
audio policy and audio flinger APIs used at track creation time.

Test: basic regression test of audio playback and capture use cases

Change-Id: I8d612e67738e120494f61e3f7c60bfd0b2c6a329