Fix FloatingWidgetView post-animation fallback background handling

The background drawables should only be restored to the original view if
the values are non-null. The original view may have had a background drawable
be a drawable, just not one that's elligible for animation. In that
case the local reference is null. We should not restore that.

Bug: 169042867
Test: manual
Change-Id: I54eaac5632e3f4f25daee2c36d7609e8b2ffedaf