Trigger predictive back animation after receiving BackNavigationInfo.

Remove onGestureStarted on the gesture down event.
Also since onThresholdCrossed can happen before startBackNavigation, it
must also trigger the predictive back animation after receiving
BackNavigationInfo

Flag: com.android.window.flags.predictive_back_delay_wm_transition
Bug: 445827520
Bug: 445335540
Test: atest BackAnimationControllerTest
Test: atest GestureNavigationFlickerTest
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:e589d3314e15062e65eeda2b58961bf4ded8a5ee
Merged-In: I74bfce4a1abcfaef1b347030293ed86e7b9ff7b0
Change-Id: I74bfce4a1abcfaef1b347030293ed86e7b9ff7b0