Don't crash if NSSL gets incomplete gesture

In *theory*, a View should always receive an ACTION_DOWN before
receiving any other touch events, but this isn't always the case in
practice. Whenever we see one of these "partial" gestures, we should
ignore it until a real one starts.

Test: manual
Bug: 146323887
Change-Id: Ic75e4873993f0a8ec3e7210fc173d1fb0bd7b71f
(cherry picked from commit d80ded29fbe1e876748ccc66579a5e82c921be65)