Check task visibility vs. root task visibility

An activity was evaluated as should-be-resumed while it
shouldn't, because the root task was always visible in
split-screen mode.

Check if the activity's task visibility instead.

Bug: 153108952
Test: atest SplitScreenTests
Change-Id: Ifa1f7b32817269d16138bbfbc7e0dc7f79c8a14a