Fix bug with selecting a secondary task as second splitscreen app via Taskbar

This patch makes it so that the correct task will be chosen when selecting a second splitscreen app via Taskbar.

Prior to this patch, the Taskbar app selection function -- which attempts to match the tapped icon to a running TaskView -- assumed that the TaskView in question was always a solo (non-grouped) Task. This resulted in the wrong app being selected for split when the desired Task happened to be the secondary app in a pair.

Fixed by checking to see if the desired app is primary or secondary, and returning the correct Task, IconView, and ThumbnailView for the split operation.

Fixes: 265244769
Test: Manual
Change-Id: Ie1122d1b49151d70dec9711fe558fba7752b7d8e