Predictive IME back animation

Known issues:
1. There is no predictive back animation played for adjustResize cases
2. This doesn't work great for adjustPan cases yet
3. A second back gesture is ignored if it is started during the post-commit (and triggerBack) phase.

Bug: 322836622
Flag: ACONFIG android.view.inputmethod.predictive_back_ime DISABLED
Test: separate CL
Change-Id: Ic8ddfffb8550cb9875e329d7a9e8b39711a512bd