Post screenshot removeWindow call if necessary

It's possible to hit the removeWindow call at a point where we've
requested that the screenshot view be attached to the window, but it
hasn't actually been added yet. In this case, the removeWindow call does
nothing (since there's no actual view to detach), but then a frame or
two later the view is attached as requested, and never removed.

This change checks in the removeWindow method whether we've requested
the view be attached, and if so waits for it to be attached and then
detaches it.

Bug: 329659738
Bug: 337229903
Test: manual, by taking many screenshots in succession + artificially
increasing the delay between requesting attachment and attaching the
view
Flag: NONE

Change-Id: I5e40e61994cde251d6c819961d88c5b0b77d9eff