Add floatingMaskView when animating to mimic bottom container.

- On expand, we add the floating mask view and translate it out at the end.
- On collapse, we translate off the mask view in the beginning once the floating mask view is added
so that we can translate it in before the actual collapsing part of the animation

bug:339850589
Test manually:
https://drive.google.com/file/d/1YNc3vq9Cb5BcbcPOHp8H3lhe6KmYBdLI/view?usp=sharing
Flag:ACONFIG com.android.launcher3.private_space_floating_mask_view STAGING

Change-Id: I7c303e6629d83408bd314886fe10113246e44dcb