A11y shortcut settings enhancement

Adding shortcut on/off switch, improving the service picker,
and adding a switch to enable the shortcut on the lock screen.

Also adjusted setting search code to avoid indexing the
accessibility shortcut aside from the main accessibility settings
page.

Bug: 35872328
Bug: 35219988
Test: Ran in a variety of conditions, ran existing settings test.
Also added basic robo test, verified existing robo tests pass.
Change-Id: I4da9bad74caf96d9c8f3640e7db5417b4ee5d602