Don't play pre-commit velocity slow-down for IME predictive back animation

For the IME predictive back animation it's better to immediately play the post-commit animation when back is invoked instead of playing the velocity slow down in BackProgressAnimator. This ensures that pre-commit and post-commit phase of the animation don't feel disconnected.

Bug: 322836622
Flag: android.view.inputmethod.predictive_back_ime
Test: Manual, i.e. verifying on device that velocity slow down is not played
Change-Id: I8cb1c7fb25a1a822271cd170467e9566d98cb6ec