finished implementing timestamp, along with refactoring services

* added VideoInfo(AbstractVideoInfo) constructor, to support later implementation for reusing info scraped into VideoPreviewInfo, into VideoInfo
* Made the Extractor class behave as a per-video object;
    - most method return values are video-specific, so it makes sense (to me) to have Extractor be stateful.
    - The only stateless methods are getVideoUrl(), getVideoId() and loadDecryptionCode(String)
* Implemented a constructor for YoutubeExtractor, which performs all initialisation work

app/src/main/java/org/schabi/newpipe/VideoPreviewInfo.java

@@ -26,10 +26,7 @@ import android.os.Parcelable;
 
 /**Info object for previews of unopened videos, eg search results, related videos*/
 public class VideoPreviewInfo extends AbstractVideoInfo implements Parcelable {
-
-
     public String duration = "";
-
     protected VideoPreviewInfo(Parcel in) {
         id = in.readString();
         title = in.readString();