Supporting windows updated per display [Part III]

1. Getting all valid displays and starting or stopping the window
tracking when accessibility service enabled or disabled.
2. Updating the window tracking when any display is added or removed.
3. Making DisplayWindowsObserver class to be plural for multiple display.

Bug: 132854721
Test: a11y CTS & unit tests
Test: manually test talkback AP at second display
Change-Id: I1691f5c2092393a6ed396623e437e5d27e0cee9a