better random

src/main/java/com/owncloud/android/utils/EncryptionUtils.java

@@ -72,6 +72,7 @@ import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.HashMap;
 import java.util.Map;
+import java.util.Random;
 
 import javax.crypto.BadPaddingException;
 import javax.crypto.Cipher;