audio policy: add support for custom mixes

Add support for custom mixes in AudioPolicyManager.

Two methods are added to register or unregister a list of custom mixes
with their attributes and format.

getOutputForAttr() and getInputForAttr() first look for a match in
registered mixes before defaulting to normal output/input selection

Remote submix device connection disconnection now takes address into
account to identify the correspnoding custom mix.

Bug: 16009464.

Change-Id: I3f1c2a485a0fb71b1f984ed0adc9b68aa971e408