Use StandardCharsets in OkHttpUtil.

okhttp3.internal.Util.UTF_8 was never meant to be used outside of
okhttp3 library; and it has been deleted in later versions.
Signal should use java.nio.charset.StandardCharsets instead.
No functional change.

Closes #12413

app/src/main/java/org/thoughtcrime/securesms/util/OkHttpUtil.java

@@ -6,13 +6,12 @@ import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.io.InputStream;
 import java.nio.charset.Charset;
+import java.nio.charset.StandardCharsets;
 import java.util.Objects;
 
 import okhttp3.MediaType;
 import okhttp3.ResponseBody;
 
-import static okhttp3.internal.Util.UTF_8;
-
 public final class OkHttpUtil {
 
   private OkHttpUtil() {}
@@ -42,7 +41,7 @@ public final class OkHttpUtil {
 
     byte[]    data        = readAsBytes(body.byteStream(), sizeLimit);
     MediaType contentType = body.contentType();
-    Charset   charset     = contentType != null ? contentType.charset(UTF_8) : UTF_8;
+    Charset   charset     = contentType != null ? contentType.charset(StandardCharsets.UTF_8) : StandardCharsets.UTF_8;
 
     return new String(data, Objects.requireNonNull(charset));
   }