Allowing activity visible behind translucent adjacent TaskFragments

An activity that behind the adjacent TaskFragments were invisible
regardless if the TaskFragments are translucent or not.

Allowing the activity to be visible in that case, just like a
TaskFragment can be visible while it is behind translucent
adjacent TaskFragments.

Bug: 203086979
Test: atest SplitActivityLifecycleTest
Change-Id: Ib487940778c856e2dd916376e98940fa17ee9012