Implement Transient Insets Animation

This implements the transient insets animation. To have InsetsPolicy run
the animation, this patch:
1. Let InsetsPolicy implements InsetsAnimationControlCallbacks;
2. Make several properties, methods and subclasses in InsetsController
   public, to let InsetsPolicy re-use them when possible.
3. Do everything necessary in InsetsPolicy to let the animation run
   correctly.

Other misc changes in this CL includes:
1. Removed some un-necessary imports;
2. Implemented two toString method, for debug purpose.

Bug: 118118435
Test: Manual test with the new Insets API test app, everything works as
      expected;
Test: go/wm-smoke
Test: atest InsetsPolicyTest

Change-Id: I6c45bd557573fcabb1a3f5aea3dcd58229280613