FocusResolver: Persist focus requests

When WM provides a focus request, the request is handled or
kept in a pending state until the window becomes visible. The request
is dropped in all other cases such as the window handle does not have
the focusable flag set. This might lead to cases where a focus request
is dropped as a result of state mismatch or synchronization issues
between wm and sf.

This fix makes the focus request persistent and better aligns with the
previous model where the focus bit was set as part of the window handle.
This means a request will be handled as long as the window is focusable
or replaced by another request. If the window toggles its focusability,
it will lose and gain focus.

Test: atest inputflinger_tests
Test: go/wm-smoke
Test: atest atest FlickerTests
Bug: 176451870
Change-Id: I790a4b69c15f595109118d49167cf9a450e67ec9