Attempt to scroll layers everytime the user drags.

This allows the user to continue scrolling a layer even after reaching the end
of it.  Only pin the fling velocity in the direction of the fling.  This is
preparation for flinging layers.

Record the inval region when updating layers so that the new layer will draw.

TODO: Keep track of whether the user started on a layer to prevent scrolling a
layer if the user drags across one.

Bug: 3022562

Change-Id: I25f84497a67dfb1e0fc424207f6c97817715501c