Implement subscriptions import/export

- Import subscriptions from YouTube and SoundCloud (all services that the extractor support)
- Import/export a JSON representation of the subscriptions
- [Minor] Remove some javax annotations in favor of the one provided by the android support library

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

@@ -89,7 +89,6 @@ public class App extends Application {
         SettingsActivity.initSettings(this);
 
         NewPipe.init(getDownloader());
-        NewPipeDatabase.init(this);
         StateSaver.init(this);
         initNotificationChannel();