Messaging: Nullability

Change-Id: I8bcde2a868a9c8989971d056be24ff4986e4f8f3

src/android/support/v7/mms/pdu/EncodedStringValue.java

@@ -20,6 +20,8 @@ package android.support.v7.mms.pdu;
 
 import android.util.Log;
 
+import androidx.annotation.NonNull;
+
 import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.io.UnsupportedEncodingException;
@@ -187,6 +189,7 @@ public class EncodedStringValue implements Cloneable {
      * (non-Javadoc)
      * @see java.lang.Object#clone()
      */
+    @NonNull
     @Override
     public Object clone() throws CloneNotSupportedException {
         int len = mData.length;