Fix bug with Tossable being used with TransitionSpring instances.

Summary:
Possible scenario (in the contextual transition demo):

1. Use a TransitionSpring with Tossable.
2. Prime Draggable with an existing gesture recognizer that is active.
3. Dismiss the photo with a drag.

Expected behavior: the photo tracks the drag's movement.
Actual behavior: the photo jumps to the final position and then starts translating.

This was happening because Draggable was reading the final value of the view before TransitionSpring had a chance to set the proper initial value, causing Draggable's initialValue to be polluted with the wrong value.

After this fix, springs will always initialize the initial state before draggable reads its initial state.

Reviewers: O2 Material Motion, O4 Material Apple platform reviewers, #material_motion, markwei

Reviewed By: O2 Material Motion, O4 Material Apple platform reviewers, #material_motion, markwei

Tags: #material_motion

Differential Revision: http://codereview.cc/D2821

src/interactions/Tossable.swift

@@ -38,10 +38,16 @@ extension Tossable: ViewInteraction {
 
     let gesture = runtime.get(draggable.nextGestureRecognizer)
 
-    runtime.add(draggable, to: reactiveView)
+    // Order matters:
+    //
+    // 1. The spring's initial velocity must be set before it's re-enabled.
+    // 2. The spring must be registered before draggable in case draggable's gesture is already
+    //    active and will want to immediately read the current state of the position property.
+
     runtime.add(gesture.velocityOnReleaseStream(in: runtime.containerView), to: spring.initialVelocity)
+    runtime.enable(spring, whenAtRest: gesture)
     runtime.add(spring, to: position)
 
-    runtime.add(gesture.atRest(), to: spring.enabled)
+    runtime.add(draggable, to: reactiveView)
   }
 }