Fix unrecoverable bad state in BackAnimationController

This CL makes BackAnimationController more robust. Should it ever enter the bad state of both touch trackers being in the initial state but mBackGestureStarted=true, it will recover from it with this bugfix.

Bug: 365461223
Flag: EXEMPT bugfix
Test: BackAnimationControllerTest
Test: Manual, i.e. verify that behaviour without fix matches bugreports and with fix recovers itself
Change-Id: I1ed592e60516e9ba173483aca1c3fdb8ba74bd59