Check is leaf task before remove its child containers

This CL change the leaf check in removeIfPossilbe().

Previously, after removeImmediately() get called, all child
containers will be removed, then isLeafTask() will always return
true due to no more child for current entity.

Bug: 177858817
Bug: 206556390
Test: CTS
Change-Id: Id645fe58442662bb26c98cba488e8d9e684ab4bc