Reduce flicker when split pair switching

Checked this issue by winscope and found home task leash under
RecentsAnimationSplitTasks will become off screen bewteen the
transition, it lead to task thumbnail invisible and wallpaper show
then cause flicker.

Home suface going to off screen is caused by we will remove previous
RecentsAnimationSplitTasks when re-create it whether going to recents
or starting split.

Fix this by seperate mSplitTasksContainerLayer to two differents
surface for going to recents and starting split cases.

Fix: 246664520
Test: manual
Test: pass existing tests
Change-Id: I492cc8b82a15cc5abec06118b54232b355e880a0