* Improved defaults for scroll view primary-ness

* Vertical scroll views default to primary:true.
* Horizontal scroll views default to primary:false.
* If a scroll view is primary and it got a non-null inherited primary
  scroll controller, it introduces a primary scroll controller inherited
  with a value of null for its descendants.

ScrollController now multiplexes writes to all registered positions;
reads of position continue to assert that only one position is
registered. Reads still require a single position.