Animate Split invocation with shell transitions

* SplitRoot1/2 and the split left/right roots, but
they also have a parent that's the top-most split root.
* We have to explicitly set the alpha on top most parent
for animations on children to be visible
* This path wasn't being used even w/ shell transitions
enabled but became active after ag/24346391

Test: Launch split from overview and workspace
Flag: None
Bug: 276361926
Change-Id: I267dccbf3402af9b0972371b856a79b7785f986c