Allow resume of parent activity that launches PiP

For an application that
- Targets pre-Q
- Launches activity A which goes into PiP mode from activity B

We prevents launching activity B from recents since we only allow
top-most visible activities for pre-Q app.

The restriction is first introduced in ag/6077827

Bug: 137085496
Test: Launch parent activity of PiP from recents
Test: atest ActivityLifecyclePipTests
Test: atest ActivityLifecycleSplitScreenTests
Test: atest WindowProcessControllerTests
Change-Id: I3a4a2c874e401a6d278d1b1b8b3fb5ee8548b8c7