Supporting windows updated per display [Part II]

Creating new class, DisplayWindowsObserver implementing
WindowManagerInternal.WindowsForAccessibilityCallback, receives
updated windows from window manager.

Bug: 132854721
Test: a11y CTS & unit tests
Test: atest AccessibilityWindowManagerTest
Change-Id: I211da59d72c284e4a87574d3fc1955f44036800a