a5: Use main-mic for handset-mic

handset-mic is used in Viber, Line, Cord, etc for VOIP messaging and chat. Using
dual-mic-inverse (through dmic-endfire) seems to be doing very bad noise
cancellation, causing murky, quiet audio in these VOIP applications. This patch
changes it to main-mic (through line-vr), which sounds much better, and matches
the HTC stock mixer_paths.

Change-Id: If8f10fd0919fb6d3ca10ad143d5d86f05d5dc2e2