audioserver: Increase hwbinder thread pool size

Use 4 threads instead of default 1, since audioserver
serves many clients.

Bug: 67203473
Test: audio still works
Change-Id: I7b191974f1a912fd8adf0096c3a1abf33a3d12fa
diff --git a/media/audioserver/Android.mk b/media/audioserver/Android.mk
index 66638e3..ee87751 100644
--- a/media/audioserver/Android.mk
+++ b/media/audioserver/Android.mk
@@ -13,6 +13,8 @@
 	libbinder \
 	libcutils \
 	liblog \
+	libhidltransport \
+	libhwbinder \
 	libmedialogservice \
 	libnbaio \
 	libsoundtriggerservice \