Stop assuming the existence of displays when tasks appear

WM shell may not receive the display added notifications when it
receives tasks appearing, so we need to wait for the display to show up
when it doesn't exist.

It's fairly difficult to test the case in real life, but it's a good
case for a unit test. Therefore this CL also makes WindowDecoration unit
tests friendly.

Bug: 236215336
Test: Caption in shell looks OK.
Test: atest WMShellUnitTests:WindowDecorationTests
Test: atest WMShellUnitTests:PipTaskOrganizerTest
Test: atest WMShellUnitTests:PipAnimationControllerTest
Change-Id: Ife0f85fdbd59b84303fc50f4ec896c53a259cec6