Prefer assertThat over assert in firebaseApp test (firebase#4997)

Author: rlazo

firebase-common/src/androidTest/java/com/google/firebase/FirebaseAppTest.java

@@ -398,7 +398,7 @@ public void testDirectBoot_shouldPreserveDataCollectionAfterUnlock() {
 
  isUserUnlocked.set(false);
  FirebaseApp firebaseApp = FirebaseApp.initializeApp(mockContext);
- assert (firebaseApp != null);
+ assertThat(firebaseApp).isNotNull();
  firebaseApp.setDataCollectionDefaultEnabled(false);
  assertFalse(firebaseApp.isDataCollectionDefaultEnabled());
  // User unlocks the device.