Handle the basic logic to show or hide accessibility floating menu

* AccessibilityFloatingMenuController will listen to the changes from AccessibilityButtonTargetsObserver and AccessibilityButtonModeObserver then handle the visibility of FloatingMenu
* Add abstract class SecureSettingsContentObserver to handle a registry of listeners of specific types and dispatch changes to child

Bug: 173940864
Test: atest AccessibilityButtonTargetsObserverTest AccessibilityButtonModeObserverTest AccessibilityFloatingMenuControllerTest
Change-Id: I07fded55340644bce84a3f17a7a4da950268d2d2