WindowInsetsAnimation: Clean up API

Fixes issues the app developers have raised with
the WindowInsetsAnimation API:

- it really makes more sense to have the Animation
  as the outer class, and the Callback nested within
- it was not obvious previously that multiple animations
  could be running at the same time. A new argument to
  onProgress now makes this abundantly clear by passing
  in the list of running animations.
- The dispatch mode really fits better as a final
  property on the callback, rather than it being
  queried once from a getter.

Also fixes lint warnings.

Fixes: 143556682
Test: make checkapi; atest WindowInsetsControllerTests
Change-Id: I8cd8faac70dd5a15d779d2c983f0a0ea5d6bbd8e