Do some code improvements

Use final where possible, annotate some methods and parameters as Nonnull and format new code to be in the 100 characters limit per line.

extractor/src/test/java/org/schabi/newpipe/extractor/services/youtube/YoutubeJavaScriptExtractorTest.java

@@ -38,7 +38,7 @@ public class YoutubeJavaScriptExtractorTest {
 
     }
 
-    private void assertPlayerJsCode(String playerJsCode) {
+    private void assertPlayerJsCode(final String playerJsCode) {
         assertThat(playerJsCode, allOf(
                 containsString(" Copyright The Closure Library Authors.\n"
                         + " SPDX-License-Identifier: Apache-2.0"),