audioserver: enable HIDL callbacks

Start hwbinder threadpool explicitly when starting services
for HIDL HALs using callbacks.
To be reverted when bug 31748996 is fixed.

Bug: 33700240

Test: check callbacks with broadcast radio HAL.

Change-Id: I504e97b6be4f95317ec04cc18df6ae545e74475d
diff --git a/media/audioserver/Android.mk b/media/audioserver/Android.mk
index c620e7c..7dc4e1d 100644
--- a/media/audioserver/Android.mk
+++ b/media/audioserver/Android.mk
@@ -14,7 +14,8 @@
 	libmedialogservice \
 	libradioservice \
 	libsoundtriggerservice \
-	libutils
+	libutils \
+	libhwbinder
 
 LOCAL_C_INCLUDES := \
 	frameworks/av/services/audioflinger \