Restructure a11y shortcut settings

Separating the shortcut on/off from the shortcut service.
Enabling the shortcut to work from the lock screen if the
user wants it to.

Bug: 35872328
Bug: 35219988
Bug: 35443593
Test: Adjusted tests for shortcut controller, manually
activated shortcut in a variety of conditions.
Change-Id: Id39d33ec62eb5f11b76b2972089167dc7d4c625b