Only use the pre-animation bounds when an animation is running and is set

- Fix issue with NaN being calculated as the saved fraction due to the wrong
  bounds being used

Bug: 109841835
Test: atest CtsActivityManagerDeviceTestCases:ActivityManagerPinnedStackTests
Test: Repeatedly launch pip into fullscreen landscape app, and ensure that
      the saved snapfraction is calculated correctly

Change-Id: I9b442cda636121164af1f3e66050894d06518439