Allow ViewGroup to split MotionEvents to multiple targets during dispatch.

Use the layout xml attribute splitMotionEvents="true" or the ViewGroup
method setMotionEventSplittingEnabled(true) to enable motion event
splitting. Rules for splitting are as follows:

* Splitting is enabled per ViewGroup. When splitting is enabled any
  MotionEvent dispatched to that ViewGroup can potentially be split
  into several and dispatched to children independently.

* Each pointer is assigned a target child view when the ACTION_DOWN or
  ACTION_POINTER_DOWN event is received. That will be the pointer's
  target until it goes up, the target returns false from onTouchEvent,
  or the MotionEvents are intercepted.

* Multiple pointers may be assigned to the same target. All pointer
  data sent to a target are bundled into a single MotionEvent. Child
  views do not need to be aware that splitting has occurred.

Change-Id: I993f838e2f6b455da9812f4742a016dfcd1c4cc9