[Hotword] Adding a field to the extension api provide info on whether the hotword enabled pref has been set. And making use of that field in the helper component extension.

BUG=345931

Review URL: https://codereview.chromium.org/175783004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@253716 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 253716