Add stub task when matching TaskView is grouped while runningTasks is not grouped

- Completely rewrote shouldAddStubTaskView to simplify the logic

Fix: 333844287
Test: With 2 split tasks, unsplit one, go to Overview, launch the other split, unsplit, go to Overview
Flag: None
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:492965326e7391c8427b6fca7875508e7a957b49)
Merged-In: I2f33f91956d485957cedf43ccd684f16d42803be
Change-Id: I2f33f91956d485957cedf43ccd684f16d42803be