Asynchronous threaded volume get/set/init

Non-blocking version of init is now being called, and asynchronous set/get of mute and volume is implmenented at PulseAudioMixer level.

The logic has been updated in SystemKeyEventListener so that the audio key behavior follows the latest doc 'Chrome OS Audio Scenarios':
  Increase volume key: unmutes aduio if muted, otherwise increases volume.
  Decrease volume key: do nothing if muted, otherwise decrease volume.
  Mute volume key: always mutes volume.
  UI is displayed after any volume key press.

Currently only the init is being called asynchronously, but the next step will be to add a call allowing asynchronous operation of the volume adjust/unmute sequence from SystemKeyEventListener, and move the ShowVolumeBubble() calls into AudioHandler so that the volume bubble will update once the calls complete on their own thread.

Patch by davej@chromium.org:
http://codereview.chromium.org/2769008/show

BUG=2910
Test=Play video or audio in ChromeOS, and press the system volume up, down, and mute keys to see they follow the above behavior.  Alternating Mute and Increment Volume presses should always bring the volume back to the same level and not creep it upwards.

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@51786 0039d316-1c4b-4281-b951-d872f2087c98

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