[Hotword] Adding UMA stats for preference state and availability of the extension.

BUG=325439, 289023

Review URL: https://codereview.chromium.org/160793002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251585 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 251585