Bug 978075 - Screen reader turn on and off announce. r=yzen

This patch does several other things:
 1. When a PrefCache with no default setting available is created, or cleared, set value to null.
 2. Removed weird margin change in announce box.
 3. Add announce method to speech presenter.
 4. Don't start and stop AccessFu implicitly in AccessFuTest.
 5. Change tests to two start/stop pairs where in one pair we activate via pref, and
     the other one via settings (TODO).
Automatic merge