Don't report displays that are going to be removed

When display is removed from system, it is immediately removed from
AM, but removal can be deferred in WM if there is an active animation
on that display. If AM then requests display ids in focus order from
WM, it might get a display that was already deleted in AM.
This CL skips displays marked for deferred removal.

Bug: 38166277
Test: DisplayContentTests#testDontReportDeferredRemoval
Change-Id: I49544963f476fc58a609094781ed46541af8a238