Prevent NPE in PulseExpansionHandler

This prevents an NPE in PulseExpansionHandler when the onTouchEvent
method is called at unexpected times. Prior to this change, the
velocityTracker could be left in an uninitialized state.

Bug: 153823196
Test: manual
Change-Id: I956cc8dbbc0fd88a5794c26ed06c18c499fc5475
1 file changed