Don't try to drag non-model-backed views.

Refactor DragStartListener to allow test confirmation of the fix.
Move DragShadowBuilder related code into DragShadowBuilder.Factory.
Further de-concrete-couple DragStartListener from handlers.
Give event handling names in DragStartListener a wee bit more explicit meaning.

Bug: 31350922
Change-Id: I5b4ff88b65c40d3ccadcda3338e547b2c5c6ac32