allow "panic button" apps to lock SMSSecure

Created by: eighthave

We've been developing "panic button" features in our apps for years, and now we are working to make a more flexible and open ecosystem of apps that can both trigger other apps, or respond to triggers. Since SMSSecure already includes a "lock" feature, it is a natural panic responder app. This setup allows for easy configuration of a single trigger action which can then trigger multiple apps.

The first commit adds support for receiving the panic trigger. The second commit wipes SMSSecure from the Recent Apps when it is responding to a panic trigger.

More info on the panic kit work here: https://dev.guardianproject.info/projects/panic/wiki

The first panic button app that sends this kind of trigger is called Ripple, and it is available here: https://github.com/guardianproject/ripple https://play.google.com/store/apps/details?id=info.guardianproject.ripple https://guardianproject.info/fdroid

I'm adding support to this app right now: https://panicbutton.io/ https://play.google.com/store/apps/details?id=org.iilab.pb

This is all code that I wrote, and you can have it under any license you want, including public domain. I waive all my copyright claims to it.