Fix Accessibility Shortcut does not show up while SystemUI restart

Root cause: SystemUI restart to initial new AccessibilityFloatingMenuController, but it won't call onUserUnlocked to turn on the floating action menu ready.
Solution: No need to check mIsAccessibilityManagerServiceReady for AccessibilityManagerService is ready to retrieve. The onKeyguardVisibilityChanged is enough to update the floating action menu visibility.

Bug: 192635136
Test: Manual testing
   1. The floating action menu is not be displayed on the lock screen
   2. The floating action menu shows on a device restart
   3. The floating action menu shows on a SystemUI restart
   4. The floating action menu shows on user switching

Change-Id: Ifdd5f71af6810342d60f2c1e70f27b171cbc4adc