Fixes https://github.com/flutter/flutter/issues/8476

More detailed list of changes in this patch:

* Replaced the didTouch special logic with more generic logic that
  uses Activities instead. Now instead when you tap down the
  Scrollable calls `hold()` which begins a HoldScrollActivity which is
  a hybrid of DragStartDetails and IdleScrollActivity and can be
  canceled. When you let go, it gets canceled and that goes ballistic.

* Make DragGestureRecognizer more aggressive about grabbing pointers,
  otherwise a second pointer in a situation with competing horizontal
  and vertical recognizers always gets taken by the other one.

* Fixed the _GestureSemantics widget to call the "down" callbacks so
  that it follows the same pattern as "real" interactions.

* Added tests for the above.

* Added a hashCode to ScrollActivity.toString (and subclasses).

* Added a toString to ScrollDragController, and include it in
  DragScrollActivity's toString.

* s/coorindator/coordinator/

* Add a comment in DragStartDetails to distinguish it from the
  otherwise identical DragDownDetails, so we're not tempted to merge
  them.