Prevent having empty task added back into TDA

Main stage will reparent specfic window mode and activity type tasks
under default task display area as its children.

When user enter some apps then press back to exit, tasks should become
out screen surface, but tasks with no activity reocrd child will
pop out again as children of default task display area while user enter
recent overview screen.

If user active stage split in such status, those tasks except the side
stage target will be reparent to main stage. After split active, if
user press back to exit app, the target task will bring to back but
other children tasks have no activity to show so home screen will
display. However, split screen cannot exit because main stage children
count still larger than 0 and both stage still visible.

For fix such issue, we should prevent empty tasks created and added
back into TDA.

Fix: 193112081
Test: Follow repro step on b/193112081 and check result
Test: atest WmTests
Change-Id: I66fcac68f6231a41b7b142307db2e7d89566cad6