AI 143815: am: CL 143758 Fixes #1749387. When a client of VelocityTracker obtains an instance from the pool, actually remove the object from the pool. Otherwise, several clients can share the same VelocityTracker which can lead to really weird side effects (including concurrency and UI issues.)

Original author: romainguy
  Merged from: //branches/cupcake/...

Automated import of CL 143815

core/java/android/view/VelocityTracker.java

@@ -58,6 +58,7 @@ public final class VelocityTracker {
             VelocityTracker vt = mPool[0];
             if (vt != null) {
                 vt.clear();
+                mPool[0] = null;
                 return vt;
             }
             return new VelocityTracker();