Miscellaneous polish for new home animation.

1. Update the starting window velocity when coming from 3-button nav or
   predictive back

Before we used an arbitrary fixed velocity which goes down to basically
0 with the old implementation, but caused a noticeable jump up in the
new one when using predictive back. Now we just pass 0, since the
predictive back framework doesn't give us the actual velocity.

2. Add the scaling home reveal to the targetless (not going back to a
   specific app icon or widget) animation and 3-button nav

Bug: 298089923
Fix: 343143876
Flag: com.android.launcher3.enable_scaling_reveal_home_animation
Test: manually tested with flag on and off
Change-Id: Ied3630a51862731fad044b68f76a45dc87f2e17b