Change a11y shortcut default lockscreen behavior

If the user hasn't chosen a preference, the shortcut
will not work on the lockscreen until the user agrees
to use the shortcut. After the user so agrees, the
shortcut will also work on the shortcut.

Bug: 70944865
Test: Adding a test for the new behavior
Change-Id: I41e1238fad43a4432cd341c6808e26ad6e155506