Remove usage of class StandardCharsets

It's possible to use this class on Android only starting with 19 API level.

Author: JinneeJ

commonmark/src/main/java/org/commonmark/internal/util/Escaping.java

@@ -1,6 +1,6 @@
 package org.commonmark.internal.util;
 
-import java.nio.charset.StandardCharsets;
+import java.nio.charset.Charset;
 import java.util.Locale;
 import java.util.regex.Matcher;
 import java.util.regex.Pattern;
@@ -78,7 +78,7 @@ public void replace(String input, StringBuilder sb) {
                     sb.append(input, 1, input.length());
                 }
             } else {
-                byte[] bytes = input.getBytes(StandardCharsets.UTF_8);
+                byte[] bytes = input.getBytes(Charset.forName("UTF-8"));
                 for (byte b : bytes) {
                     sb.append('%');
                     sb.append(HEX_DIGITS[(b >> 4) & 0xF]);

commonmark/src/main/java/org/commonmark/internal/util/Html5Entities.java

@@ -4,7 +4,7 @@
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.InputStreamReader;
-import java.nio.charset.StandardCharsets;
+import java.nio.charset.Charset;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.regex.Matcher;
@@ -43,7 +43,8 @@ public static String entityToString(String input) {
     private static Map<String, String> readEntities() {
         Map<String, String> entities = new HashMap<>();
         InputStream stream = Html5Entities.class.getResourceAsStream("entities.properties");
-        try (BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(stream, StandardCharsets.UTF_8))) {
+        Charset charset = Charset.forName("UTF-8");
+        try (BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(stream, charset))) {
             String line;
             while ((line = bufferedReader.readLine()) != null) {
                 if (line.length() == 0) {