Added an App Widget and made it available to the lock screen by setting "keyguard" in android:widgetCategory. Documentation on how to do it: https://developer.android.com/guide/topics/appwidgets/index.html

It's a little tricky to actually add the widget to the lock screen. It doesn't seem possible when a PIN has to be entered for now. Without security (i.e. swipe unlock) a lock screen widget can easily be added.