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
diff --git a/tests/src/com/android/launcher3/util/rule/SimpleActivityRule.java b/tests/src/com/android/launcher3/util/rule/SimpleActivityRule.java
index 33a6cf9..1dbba6a 100644
--- a/tests/src/com/android/launcher3/util/rule/SimpleActivityRule.java
+++ b/tests/src/com/android/launcher3/util/rule/SimpleActivityRule.java
@@ -64,6 +64,7 @@
                 mBase.evaluate();
             } finally {
                 app.unregisterActivityLifecycleCallbacks(this);
+                mActivity = null;
             }
         }