Use HEADSET Tx if mic_type is digital and Rx device is HEADPHONES

Some devices require a different tx device when in_call and
headphones are inserted. While DMIC Endfire is the correct Tx
device for Earpiece Rx, choosing DMIC Endfire when Rx is
HEADPHONES will result in inaudible voice. This patch will only
affect devices that choose 'digital' as their mic_type via
prop 'persist.audio.handset.mic'

Example device: LG Optimus G Pro E980

Change-Id: Ia80301f12e945fcc8841e9053f399ff64b9f6d45