Don't allow other apps to open the panic prefs activity

The branch prevents other apps from opening PanicPreferencesActivity.

Briar still shows up in Ripple's list of responders (presumably because we listen for TRIGGER intents), and there's no change to the way panic actions are handled. Touching Briar in Ripple's list of responders has no effect (it used to launch PanicPreferencesActivity). The text next to Briar in Ripple's list of responders says "App hides when triggered" (it used to say "EDIT"). This text is wrong, but it seems to be a default supplied by Ripple.

Depends on !501 (merged).

Fixes the second part of #909 (closed).