Removes shortcut target when an a11y service is unbound

The accessibility button and shortcut target is updated by
the Settings app when the user turn on/off the a11y service.
It worked fine before an a11y service disables and turns off
itself using disableSelf api. The accessibility button and
shortcut settings can not get update in this case. Removes
shortcut target if an a11y service is unbound to fix this
issue.

Bug: 156237174
Test: atest AccessibilityShortcutTest
Change-Id: Ie15f7fe06644bd35d30d87383b25df81bf29056d