Make accessibility window events more granular

TYPE_WINDOWS_CHANGED events have meant that "something"
changes with the system windows, so a single event would be
fired for a single small window change or a bunch of window
changes. Services were required to cache the windows and
compare the old ones to the new ones to find out what
changed.

Since AccessibilityEvents are intended to communicate
changes to the UI, this CL sends one TYPE_WINDOWS_CHANGED
for each window that has changed, and provides a way to
get the source of the changing window. It also adds a
windowChangeType field, which contains flags to indicate
what exactly has changed for the window.

Bug: 62231686
Test: Run all a11y unit and cts tests
Change-Id: I6c48e74da26be5ea485ac114a37f3c404a74940e