alsa_sound: Add support for "Voice Earpiece Extra Gain" mode

Uses "Voice Earpiece Extra Gain" mode instead of "Voice Earpiece" during
voice calls when persist.audio.voc_ep.xgain=1.

Change-Id: I9f00a81687fa9605c68308a74e0ffaaea7e698fc

Conflicts:
	alsa_sound/ALSADevice.cpp