Use @SuppressWarnings for checkstyle suppressions & warnings

It's better to use @SuppressWarnings instead of the suppressions file, so that the warning suppression is at the place where it acts.

app/src/main/java/org/schabi/newpipe/streams/Mp4FromDashWriter.java

@@ -142,6 +142,7 @@ public class Mp4FromDashWriter {
         outStream = null;
     }
 
+    @SuppressWarnings("MethodLength")
     public void build(final SharpStream output) throws IOException {
         if (done) {
             throw new RuntimeException("already done");

app/src/main/java/org/schabi/newpipe/streams/WebMWriter.java

@@ -121,6 +121,7 @@ public class WebMWriter implements Closeable {
         clustersOffsetsSizes = null;
     }
 
+    @SuppressWarnings("MethodLength")
     public void build(final SharpStream out) throws IOException, RuntimeException {
         if (!out.canRewind()) {
             throw new IOException("The output stream must be allow seek");