[FEATURE] Use Intent to call external screensaver app

[theothernt]

Your feature request. Please describe. Currently, you can set a different webpage URL as the screensaver after a timeout.

I already have a 3rd party screensaver app I'd like to call. I would like a setting to specify an activity to call instead of a URL, for example...

com.neilturner.aerialviews/.ui.screensaver.TestActivity

I would be happy to create a PR for this feature if it sounds like a good idea?