Fixing activity being referred after destroying.

This is perhaps not a leak, but it conflicts with how StrictMode
activity leak detection works.

Bug: 139137636
Change-Id: I1608e348d8b238142c42ed81ae125fd07ef6b673

tests/src/com/android/launcher3/util/rule/SimpleActivityRule.java

@@ -64,6 +64,7 @@ public class SimpleActivityRule<T extends Activity> implements TestRule {
                 mBase.evaluate();
             } finally {
                 app.unregisterActivityLifecycleCallbacks(this);
+                mActivity = null;
             }
         }