Fixes long press the a11y shortcut no response

An a11y service targeting sdk version > Q and requesting a11y button
should be turned on by the Settings when it's assigned to the a11y
shortcut by the user. An a11y button callback is sent to the a11y
service when the user long press the a11y shortcut.

This issue happened when an disabled a11y service is assigned to the
a11y shortcut, and upgraded to the target sdk version > Q. Framework
fails to send the callback and no response when user long press the
shortcut.

Turns on the a11y service and show up a toast to fix this issue.

Bug: 153517972
Test: atest AccessibilityShortcutControllerTest
Change-Id: Ie4fc9bbeaf08ba674cfe6382eab214b2a15c004c