added: documentations on lifecycles for FailedMediaSource and LoadedMediaSource.

fixed: onPlaybackSynchronize to rewind when not playing, which was incorrectly removed in previous commit.
fixed: sonar and checkstyle issues.

app/src/main/java/org/schabi/newpipe/player/mediaitem/PlaceholderTag.java

@@ -1,6 +1,5 @@
 package org.schabi.newpipe.player.mediaitem;
 
-import org.schabi.newpipe.extractor.stream.StreamInfo;
 import org.schabi.newpipe.extractor.stream.StreamType;
 import org.schabi.newpipe.util.Constants;
 
@@ -74,16 +73,6 @@ public final class PlaceholderTag implements MediaItemTag {
         return StreamType.NONE;
     }
 
-    @Override
-    public Optional<StreamInfo> getMaybeStreamInfo() {
-        return Optional.empty();
-    }
-
-    @Override
-    public Optional<Quality> getMaybeQuality() {
-        return Optional.empty();
-    }
-
     @Override
     public <T> Optional<T> getMaybeExtras(@NonNull final Class<T> type) {
         return Optional.ofNullable(extras).map(type::cast);