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
diff --git a/services/audioflinger/AudioFlinger.h b/services/audioflinger/AudioFlinger.h
index a16fa94..ef218fa 100644
--- a/services/audioflinger/AudioFlinger.h
+++ b/services/audioflinger/AudioFlinger.h
@@ -104,6 +104,7 @@
 class AudioBuffer;
 class AudioResampler;
 class DeviceHalInterface;
+class DevicesFactoryHalCallback;
 class DevicesFactoryHalInterface;
 class EffectsFactoryHalInterface;
 class FastMixer;
@@ -836,6 +837,7 @@
                 DefaultKeyedVector<audio_module_handle_t, AudioHwDevice*>  mAudioHwDevs;
 
                 sp<DevicesFactoryHalInterface> mDevicesFactoryHal;
+                sp<DevicesFactoryHalCallback> mDevicesFactoryHalCallback;
 
     // for dump, indicates which hardware operation is currently in progress (but not stream ops)
     enum hardware_call_state {