Optimize system bar switching animations

- Fix a bug where the wrong time was used when the activity was in
overview, but overview wasn't actually visible.
- Fixing this looks pretty bad. Introduce a pre-delay since our
animations decelarate heavily at the end.
- Consolidate logic between swipe-up and swipe-down. Take a middle
value that is most in line with the other animations.

Test: Open/close apps from home and recents with and without
swiping.
Fixes: 109906294

Change-Id: I9cd5568c60fe7281d0f71a75981a3003770e659e