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
Merged-In: Iac4821b3daf993da9fc309b7b105db98ee4b89ce
Change-Id: I0d2194c4145927b21df5858a3787f8c210f2794c
(cherry picked from commit fe319172)