hal: Add support for audio power policy handling

* Add PowerPolicyClient class to receive policy change notifications

* Add PowerPolicyClient launcher to join ABinder threadpool

* Add functionality to launch powerpolicy extn in a new thread

Change-Id: Ic9349ca7c94fe481b2d7a74536f97e32a7b0fe8e
Signed-off-by: Tahir Dawson <dawson@codeaurora.org>