Detect when pointer has stopped moving.

Some input devices do not generate ACTION_MOVE events while all
pointers have stopped, thereby lulling the VelocityTracker into
a false sense of complacency.  Before handling the following sample,
reset the VelocityTracker state so as not to be influenced by
earlier samples before the pointer stopped.  The velocity after
stopping is assumed to be discontinuous.

Bug: 6413587
Change-Id: I6387bc036ff141d083d3d17a89e37eeaa3188349