[DO NOT MERGE] ActivityEmbedding: hide embedded TaskFragment between

adjacent TFs

An old secondary TaskFragment can be in middle of the primary and new
secondary TaskFragments. We don't want to show it even if it has a
different bounds from the new secondary.

Fix: 215467839
Test: atest SplitActivityLifecycleTest
Change-Id: I85dccdc4a931eda7d53c91d288ad19b6ef865dc9