Removes the dependency between AudioManager and PluginManager.

Now AudioManager can listen the audio without hotword recognition.
The mean volume level would be useful to the speech UI.

BUG=313904
R=xiyuan@chromium.org
TEST=manually

Review URL: https://codereview.chromium.org/92863004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238753 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238753