Add placeholder divider view when starting split tasks

* This prevents launcher underneath from peaking through
while the split apps are loading

Bug: 299640096
Test: Launched from recents, home and all apps
Doesn't seem to affect small screen since recents scrolls
away and the scrim is only left

Change-Id: I32e394a0bc361489473ee657161c8f3bcbf1e422
(cherry picked from commit cdaabc61)
Merged-In: I32e394a0bc361489473ee657161c8f3bcbf1e422