Introduces 'STOPPING' state to the speech manager.

When the app-list is closed when 'RECOGNIZING' state, it will
stops both AudioManager and SpeechRecognitionManager, but
SpeechManager will restart AudioManager on speech recognition's end.
So it may keep waiting for the hotword which would be bad.

BUG=None
R=xiyuan@chromium.org

Review URL: https://codereview.chromium.org/92583003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238833 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238833