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/res/menu/main_fragment_menu.xml

@@ -5,6 +5,7 @@
     <item
         android:id="@+id/action_search"
         android:icon="?attr/search"
+        android:orderInCategory="1"
         android:title="@string/search"
         app:showAsAction="always"/>
 </menu>