Insets Animation: Add CancellationSignal to controlWindowInsetsAnimation

Fixes an API deficiency where the request to control the animation
could not be cancelled until it has succeeded (or failed). To address that,
we return a CancellationSignal.

Bug: 143556682
Test: make checkapi; atest InsetsControllerTest
Change-Id: I03d1bcc7013c10d2deac7b1059b99894b7103230