WindowInsetsAnimationController: Fix minor API issues

- make sure we only dispatch the controller in onCancelled after the app has seen onReady
- return a linearly interpolated getFraction() if there is no interpolator instead of -1

Bug: 118118435
Test: atest WindowInsetsAnimationControllerTests
Change-Id: Iccd0b6246b4cdc250f3111409821c1dac53c694e