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