audio policy: audio sessions on input descriptors

In preparation for concurrent capture, add support for multiple
audio sessions per input stream.

Each session keeps its own properties, open and active reference
counting.

No functional change for now: still one session per input and one active
input at a time.

Bug: 18815985.
Bug: 22702906.

Change-Id: I915a65989a7fd0d3cbe2fcf5a0aee2ea0df5f4f5