Settings for new accessibility shortcut.

Repurposing shortcut setting to make it configurable.

Bug: 30160335

Test: Verified that the setting dialog appears with both
default and newly added accessibility services. Toggled
services on and off, and verified that nothing happened
when the shortcut was disabled.
Change-Id: I14d00109212bdd4b7919b2d51f702c8f118007fa