Linux and Windows were already reporting audio output device changes as
DEVTYPE_AUDIO_CAPTURE.

This CL basically renames DEVTYPE_AUDIO_CAPTURE to DEVTYPE_AUDIO and
makes the CrOS device monitor to report audio-output device changes in
addition to audio-capture devices.

The only platform that needs further work is Mac, which currently does
not report device changes for output-only devices such as USB
speakers.

BUG=587396

Review-Url: https://codereview.chromium.org/2326623003
Cr-Commit-Position: refs/heads/master@{#419136}