Default to using SplitSelectSource drawable if TaskView icon drawable is null

* Alternative solution could be to set
onTaskViewVisibilityChanged(true) for the taskView that is about
to be dismissed so it loads it's taskIcon/thumbnail back from the cache
* However, that does still leave us open to race conditions (even though
we can be reasonably confident the icon is probably in the cache)
* Also made other changes to allow already public fields on some classes
to be mockable for unit testing

Fixes: 275267738
Test: Tested with fullscreen task at end of overview,
GroupedTaskView at end of overview,
Initiating split from home,
Initiating split from overview actions,
Initiating split from overview app icon

Change-Id: Ic9059c93c07b90f61c9f418d5d36d6ba201ff96a