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: I22182d90b0057df67fbd6d20785da23d6c17dcf3