i9305: increase back/main mic gain

Change-Id: I1da30def5b57380481adf746c15be52061efc2ee
i9305: Force the use of both microphones to allow stereo recording

Force the use of both AUDIO_DEVICE_IN_BACK_MIC and AUDIO_DEVICE_IN_BUILTIN_MIC if the
device AUDIO_DEVICE_IN_BACK_MIC is selected to allow stereo recording

Change-Id: I0ad230f2c63ba96a87a642dc92ad33f4eb807367