Keep a separate accessibility focus for proxy-ed displays.

Proxy display a11y focus changes will not clear non-proxy displays,
and vice-versa.
The logic for a proxy display generally follows the logic for a
non-proxy display when deciding when to update
windows#isAccessibilityFocus/send WINDOWS_CHANGE_ACCESSIBILITY_FOCUSED.

Bug: 254545943
Test: manual check with TalkBack on and a proxy-ed Exo
VirtualDisplay. Both can retain and set their own a11y focus.
atest AccessibilityDisplayProxyTest,
atest AccessibilityManagerServiceTest

Change-Id: I76f72acf4c72ae3ce23cd2240430de3e844d2e4b