Supporting windows updated per display [Part I]

1. Making WindowsForAccessibilityObserver class to be plural for
supporting multi-display.
2. Adding the display information when WindowManagerService informs the
window changes to Accessibility Controller.

Bug: 132854721
Test: a11y CTS & unit tests
Change-Id: I0efba7d924b24fcab85c6bb333687cc4cefa890f