A11yManagerService turns off button targets for services whose packages have forcibly stopped.

Change involves reorganizing relevant code so that it is testable.
Exposed code is documented.

Feature flag:
-namespace: accessibility
-flag: com.android.server.accessibility.disable_continuous_shortcut_on_force_stop

Bug: 198018180
Test: atest A11yManagerServiceTest
Change-Id: Ic54591590af402115607f2d0f524276d1ec365f7