Check proxy status when determining whether to track windows

Marking DisplayWindowsObserver as belonging to a proxy when the
proxy is registered will not work since the observer may be removed
immediately (the proxy service info may not be propagated yet, so
a11y is off and tracking is stopped)

Mark the observer when the a11y system decides to track windows.

Test: manual check with sample app, atest AccessibilityDisplayProxy
Bug: 263375218
Change-Id: I3042b0c77b341251c5590d9e9f2fb9677574d128