Mute audio when volume is zero on Android.

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.

BUG=263399
R=henrika@chromium.org, qinmin@chromium.org, tommi@chromium.org

Review URL: https://codereview.chromium.org/93233003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@239007 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 239007