add new line to the end of BannerViewHelper.java

Author: DellaBitta

admob/src_java/com/google/firebase/admob/internal/cpp/BannerViewHelper.java

@@ -621,4 +621,4 @@ public static native void completeBannerViewFutureCallback(
 
   /** Native callback to notify the C++ wrapper that a state change has occurred. */
   public static native void notifyStateChanged(long nativeInternalPtr, int changeCode);
-}
+}