Replace explicit string checks whether a playlist íd is a certain YouTube Mix...

Replace explicit string checks whether a playlist íd is a certain YouTube Mix type with calling the dedicated methods

extractor/src/main/java/org/schabi/newpipe/extractor/services/youtube/YoutubeParsingHelper.java

@@ -226,20 +226,20 @@ public class YoutubeParsingHelper {
      * @throws ParsingException If the playlistId is a Channel Mix or not a mix.
      */
     public static String extractVideoIdFromMixId(final String playlistId) throws ParsingException {
-        if (playlistId.startsWith("RDMM")) { //My Mix
+        if (playlistId.startsWith("RDMM")) { // My Mix
             return playlistId.substring(4);
 
-        } else if (playlistId.startsWith("RDAMVM") || playlistId.startsWith("RDCLAK")) { //Music mix
+        } else if (isYoutubeMusicMixId(playlistId)) { // starts with "RDAMVM" or "RDCLAK"
             return playlistId.substring(6);
 
-        } else if (playlistId.startsWith("RMCM")) { //Channel mix
-            //Channel mix are build with RMCM{channelId}, so videoId can't be determined
+        } else if (isYoutubeChannelMixId(playlistId)) { // starts with "RMCM"
+            // Channel mix are build with RMCM{channelId}, so videoId can't be determined
             throw new ParsingException("Video id could not be determined from mix id: " + playlistId);
 
-        } else if (playlistId.startsWith("RD")) { // Normal mix
+        } else if (isYoutubeMixId(playlistId)) { // normal mix, starts with "RD"
             return playlistId.substring(2);
 
-        } else { //not a mix
+        } else { // not a mix
             throw new ParsingException("Video id could not be determined from mix id: " + playlistId);
         }
     }