Attempts fixing test flakiness

There is a race condition between the test and the launched
activity, which could result in NullPointerException and
break the test.

Bug: 289865322
Test: atest ActivityThreadTest over 500 times on cf
Change-Id: I1e13c9c1dd796296263de0984675311866ba61f3

core/tests/coretests/src/android/app/activity/ActivityThreadTest.java

@@ -884,12 +884,13 @@ public class ActivityThreadTest {
             mConfig.setTo(config);
             ++mNumOfConfigChanges;
 
-            if (mConfigLatch != null) {
+            final CountDownLatch configLatch = mConfigLatch;
+            if (configLatch != null) {
                 if (mTestLatch != null) {
                     mTestLatch.countDown();
                 }
                 try {
-                    mConfigLatch.await(TIMEOUT_SEC, TimeUnit.SECONDS);
+                    configLatch.await(TIMEOUT_SEC, TimeUnit.SECONDS);
                 } catch (InterruptedException e) {
                     throw new IllegalStateException(e);
                 }