Plumb the notification about audio HAL services update to APM

AudioFlinger registers a callback with libaudiohal to receive
notifications when new HAL services get registered. It relays
the notification to AudioPolicyManager via AudioSystem /
IAudioPolicyService interface.

Because AF / APM only interact via Binder interfaces and APM's
interface gets registered later than AF's, the notification
from AF is made asynchronous.

Bug: 149854039
Test: audio test on a regular phone configuration
      audio test on a phone with MSD audio HAL module
Change-Id: I158e941b8f75e2a4614b9d84ca798b0f1f47aa6a
Merged-In: I158e941b8f75e2a4614b9d84ca798b0f1f47aa6a