Clear UI when mailbox is wiped remotely

When the mailbox is wiped remotely by Briar we should clear the UI (perhaps using something like Briar's HideUiActivity). Currently the UI stays visible. Tapping the home button and then relaunching the app shows the status screen with "Mailbox is running" (on Android 5 at least - on Android 12 I think the home button would behave differently).