Add audio.a2dp.default to generic system image

audio.a2dp.default is a system module. Generic system image includes
audio.a2dp.default to support A2DP if board has the capability.

Bug: 62649731
Test: build and check the file existance in system image
Change-Id: Iac4821b3daf993da9fc309b7b105db98ee4b89ce
diff --git a/target/product/treble_common.mk b/target/product/treble_common.mk
index 5479c45..d5366d9 100644
--- a/target/product/treble_common.mk
+++ b/target/product/treble_common.mk
@@ -205,3 +205,9 @@
     frameworks/av/services/audiopolicy/config/r_submix_audio_policy_configuration.xml:system/etc/r_submix_audio_policy_configuration.xml \
     frameworks/av/services/audiopolicy/config/audio_policy_volumes.xml:system/etc/audio_policy_volumes.xml \
     frameworks/av/services/audiopolicy/config/default_volume_tables.xml:system/etc/default_volume_tables.xml \
+
+# Bluetooth:
+#   audio.a2dp.default is a system module. Generic system image includes
+#   audio.a2dp.default to support A2DP if board has the capability.
+PRODUCT_PACKAGES += \
+    audio.a2dp.default