Set GroupedTaskView child thumbnails in onMeasure

* Also add indicator if split was originally started
in portrait of landscape, so we know which dimension
of the divider bar to use if user goes to overview
and then rotates device (horizontal divider vs
vertical)

Fixes: 199461137
Test: Swipe to overview with staged split
Rotate device with and without live tile
Doesn't overlap with overview actions

Change-Id: I8b7f104f16d5b7265828f1b3d98ba3426b28d44f