[Android] Set webaudio to be muted when volume is set to 0

We are using Communication mode for audio backend on Android. By design,
the platform voice volume never goes to zero. This patch uses ContentObserver
to listen to volume change. When volume is set to zero, AudioManagerAndroid
will mute all its output streams.

The patch is back ported from chromium trunk, please see:
https://codereview.chromium.org/93233003
Please remove this patch if chromium is rebased to version 33.

BUG=https://crosswalk-project.org/jira/browse/XWALK-622
Merge pull request #96 from yinli/mute-webaudio

[Android] Set webaudio to be muted when volume is set to 0