Correctly handle intercepted touch events in FastScroller

Previously we'd return true from onIntercept and assume the DOWN event
was handled, then onTouch would return false and the host view would
receive the DOWN event. Now we consume the event in onTouch.

Bug: 19080751
Change-Id: I1bd35b4c25a0760f4248ade443d09b8ca4b71e7c