[Hotword] Making IsAvailable convenience function in hotword_service_factory to make calling simpler in other classes.

Dependent on 152513003 getting checked in first.

BUG=325439, 289023

Review URL: https://codereview.chromium.org/138073014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248942 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248942