Support derived target update of device manifest

If EMULATOR_VENDOR_NO_SOUND is true, the derived target can specify its
own audio implementation. However, this requires specifying the audio
effect implementation in the device manifest file. Allow the
DEVICE_MANIFEST_FILE to be added to in the derived target.

Test: make
Change-Id: I3ffa25639d2901d9e9d4370d90cc09d300f1cc60
2 files changed