Fix drop to enter split not working

Sometimes when dropping app icon to enter split screen, it'll exit split
screen immediatlly because it evicts newly launched task before the task
considered to be visible. Update to collect child tasks to evict before
launching activity.

Fix: 203610931
Fix: 203636251
Test: atest WMShellUnitTests
Change-Id: If070fb5b3eb0b3fc178122d38b2dce3072621449