Fix NullPointerException in GestureDetector

NPE can be thrown from GestureDetector on GestureDetector#cancel which
can crash an app as VelocityTracker object is not checked for null
before calling VelocityTracker#recycle.
This change fixes the issue by checking for null VelocityTracker before
calling VelocityTracker#recycle on GestureDetector#cancel.

Test: Precautionary fix to prevent crashes, no test was run

Change-Id: I805ac4a19fbd766a9d0f17732b158f1d1e61aae6