Fix crash in WindowDecoration

Ife0f85fdbd59b84303fc50f4ec896c53a259cec6 introduced a workaround
of registering a display listener when the display a task is on
isn't available when the task appears. However, the logic could
go into an infinite loop of:

(i) A display listener gets added
(ii) onDisplayAdded gets invoked, which triggers relayout
(iii) relayout() calls getDisplay() but it returns null, so goes
back to (i)

This change simply avoids this by making (i) asynchronous. Even
in the intended usage, we need to wait for the display to be added
anyway.

Flag: EXEMPT simple bug fix
Bug: 415631133
Test: CompatChangeTests#testSizeCompatDoesNotRestartWithDisplayMove
Test: WMShellUnitTests:WindowDecorationTests
Change-Id: I5de9302360a5f31dd6842f850a95163a3132f98b