Prevent extraneous saving of pinned snap fraction.

- When the PIP is dismissed offscreen, the pinned stack can receive
  onConfigurationChanged (when its windowing mode is set back to
  fullscreen) after the stack is hidden.  In this case, the saved snap
  fraction will be incorrect leading to a wrong animation on the next
  PiP animation. We don't expect the snap fraction to be saved in this
  case since the user is not expanding the PiP.
- Also add the animating-bounds state to the proto for use in the cts
  tests.

Bug: 73775460
Test: atest CtsActivityManagerDeviceTestCases:ActivityManagerPinnedStackTests
Change-Id: I30398ecf11de045d11a95282b8203e4d2d344ae9