Fix PB animation is not finished normally.

If two back gestures occur very close to each other and the first
animation didn't ready before second gesture, the animation for the
first gesture may not finish properly because there won't play post
commit animation. Checking the state from gesture tracker should be
more reliable than checking current state.

Flag: EXEMPT bugfix
Bug: 377479922
Test: atest BackAnimationControllerTest
Change-Id: I5399116ed29da5d87dd735cf54fa2efe982d4251