Adjust a11y shortcut settings based on UX input

Disabling the shortcut if no service is configured, so we
users won't see an "On" unless the shortcut will have an
effect.

If no service is configured and the user tries to enable
the shortcut, the service picking UI is shown. If the user
chooses a service, the shortcut turns on. If not, the
shortcut remains off.

Service selection is disabled when the shortcut is disabled
(except for the specific case above).

Lock screen behavior selection is disabled when the shortcut
is disabled.

Also updating a couple of strings based on UX input.

Adding an AOSP header to a file that somehow got checked in
without one.

Fixes: 37443184
Fixes: 38030853

Test: Manually went through the various combinations of
shortcut on and service enabled. I turned off the default
selection of TalkBack as the shortcut service to ensure
that we show a dialog when no service is selected.

Change-Id: Iac5cff12edea91a496fc81781518db4ae49ef334