AudioMixer: Expand mono track to multi-channel

Extend MONO_HACK to multi-channel output.

Mono track will have only one channel output on multi-channel devices.
Since ARC++ exposed a 4-channel output device and downmix track in
ChromeOS, we need to add this feature to support mono track sample
expansion.

Add support for both re-sample and non-resample path.
For resample path, we need to add MIXTYPE_STEREOEXPAND in
AudioMixerOps.h since AudioResampler will upmix mono track to stereo
track.

Bug: 120222604
Bug: 112341269
Bug: 117116052
Bug: crbug.com/890560
Test: Play mono tracks without re-sampling on ARC++
Test: Play mono tracks with re-sampling on ARC++
Test: Play normal stereo tracks on ARC++
(cherry picked from commit 9b79e0752e6536c31430aa31838a9de1b7b56f9f)

Change-Id: I51f5914c41dd0196db9c6a2e1a99b44e5d87c0d6