Don’t dismiss split screen while were occluded

Split screen will be dismissed right after both sides of the split
were occluded when shell transition enabled. This CL removes the
dismissed logic which makes the behavior the same as the tm-qpr
branch.  Also change the evict child task process first while
startTask to make sure tasks are in split.

Bug: 268494905
Test: 1. Activate a split 2. Launch full-screen task to occlude split
Test: atest SplitTransitionTests, atest WMShellFlickerTests
Change-Id: Ia8a484e1cd4a9570c5484cb1009b2bd09a61aa70