Observer for the accessibility button mode and notify its listeners for the changes

* Update accessibility button state (visibility) in navigationbar when
accessibility button mode listener receive the changes.

Cherry picked from commit 2c60295b

Bug: 173938894
Bug: 173940864
Test: atest AccessibilityButtonModeObserverTest
Change-Id: I03b961c95a20e4698491443012beb3ab73333199
Merged-In: I03b961c95a20e4698491443012beb3ab73333199