-Added schema for local playlist and stream statistics.

-Added normalized schema for stream history.
-Added managers for specialized database access for stream and local playlist.

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

@@ -28,4 +28,10 @@ public final class NewPipeDatabase {
 
         return databaseInstance;
     }
+
+    @NonNull
+    public static AppDatabase getInstance(Context context) {
+        if (databaseInstance == null) init(context);
+        return databaseInstance;
+    }
 }