Separate code paths to launch split from existing pair vs new pair

* Launching existing pair doesn't rely on any existing
data/state in SplitSelectStateController or SplitSelectDataHolder
* Consolidate launchTask*() methods in GroupedTaskView

Test: TaskThumbnailViews measured correctly, animation
correct on small+large screen, initiating split
still working
Fixes: 274540670
Flag: none needed

Change-Id: Id359f53ac5f2bba19948d4ae3231092fb90cc7c5