Expose A11yDisplayProxy#findFocus

A proxy can retrieve input or accessibility focus.
- Make sure a proxy and non-proxy user cannot obtain each other's nodes.
- Remove ACTIVE_WINDOW code when finding focus, since ACTIVE_WINDOW is
  not used.

What this CL does not do:
- separate input focus between proxy and non-proxy users. This can't be
  done properly until V
- separate a11y focus between proxy and non-proxy users. This will be in
  a future CL.

Bug: 264594384
Test: atest AccessibiltiyDisplayProxyTest
Change-Id: Iefd88806a5a33deb7ef2ec2460ee79167c82519d