audio policy: volume control reorganization

Output volume and routing control by AudioOutputDescriptor
is reorganized to prepare hardware source volume and routing
control.
AudioOutputDescriptor contains all volume, device and activity
state common to software (audio flinger mixers) and
hardware sources (tuners, A2DP, HDMI).
A new class SwAudioOutputDescriptor is derived from
AudioOutputDescriptor and is specific to software sources.

Low level routing and volume control methods receive an
AudioOutputDescriptor parameter instead of an IO handle.

mPrimaryOutput is now an AudioOutputDescriptor.

Change-Id: Ie90943ee3102cdb8adf89fdd2addd2c279b1e5bf