Added missing import java.nio.charset.StandardCharsets

Author: leonard

aws-android-sdk-cognitoauth/src/main/java/com/amazonaws/mobileconnectors/cognitoauth/util/Pkce.java

@@ -20,6 +20,7 @@
 import android.util.Base64;
 
 import java.nio.charset.Charset;
+import java.nio.charset.StandardCharsets;
 import java.security.MessageDigest;
 import java.security.SecureRandom;