The old GetDeviceChannels() method returns the total number of channels
on a device, not necessarily how many its configured to use. For devices
which allocate ranges of channels to different behaviors (mirroring, etc)
this is completely broken.

Instead use kAudioDevicePropertyPreferredChannelLayout, which seems to
give us the right information in both the input and output cases. This
sometimes requires us to make an additional call to convert the given
information into something we can easily parse.

BUG=671308
TEST=manual; configure 6 channel virtual device as stereo.
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2695633006
Cr-Commit-Position: refs/heads/master@{#453023}