Code style improvements

Use final where possible in YoutubeStreamExtractor and do some other code style improvements

Author: AudricV

extractor/src/main/java/org/schabi/newpipe/extractor/exceptions/AgeRestrictedContentException.java

@@ -4,6 +4,7 @@ public class AgeRestrictedContentException extends ContentNotAvailableException
     public AgeRestrictedContentException(final String message) {
         super(message);
     }
+
     public AgeRestrictedContentException(final String message, final Throwable cause) {
         super(message, cause);
     }

extractor/src/main/java/org/schabi/newpipe/extractor/exceptions/ExtractionException.java

@@ -32,4 +32,4 @@ public ExtractionException(final Throwable cause) {
     public ExtractionException(final String message, final Throwable cause) {
         super(message, cause);
     }
-}
+}