Add nested pre-fling for nested scrolling

Nested pre-fling allows a nested scrolling parent to consume a fling
operation before the child view does. If a parent has been consuming
nested pre-scroll events in a particular direction, this is generally
a good indication that it should also consume the resulting fling at
the end.

Bug 15538504

Change-Id: I88e8753a96c9b41815c3be530cafab8345164e64