Fades out Assistant when entering Overview.

This makes the transition smoother, since
the Assistant goes away at the end of it.

Note this only applies to the transition
while an app other than Launcher is live.

Before: https://drive.google.com/open?id=18SOWTtTUzt19TReAYnofQLIShJ2I-odP
After: https://drive.google.com/open?id=1hqsikXSlCnSvNg0ZQYHxP-OIbS5yt9aW

Bug: 136282913
Change-Id: Ia53c2407c82725c776f27b0d6e80a34554b4a303
Merged-In: Ia53c2407c82725c776f27b0d6e80a34554b4a303
Tested: Manually