Making activity invisible when occluded by one of the adjacent TF

Multiple TaskFragments can be added in a leaf Task for activity
embedding use case. Unlike split-screen use case, the adjacent
TaskFragments are not always positioned next to each other.
There could have a TaskFragment positioned in between two adjacent
TaskFragments.

Bug: 204185619
Test: atest SplitActivityLifecycleTest

Change-Id: I27295a8176c61044bd8e92d70cb1aa422e3e88f6