WindowInsetsAnimationControllerTests: wait for animations to finish

If we register the VerifyingCallback animation listener mid-animation,
it will receive onEnd callbacks for things it did not see the start of;
to avoid that, we just wait for animations to settle down first.

Fixes: 175745628
Test: atest WindowInsetsAnimationControllerTests
Change-Id: I332462b9e204963a866e1dea434daf0d710e8885