Show a warning on Android 4 that Briar will expire

Show a snackbar on Android 4, similar to the existing expiry snackbar for debug builds, warning the user that the app will expire on a certain date and they will need to upgrade to a newer device and create a new account.

If the explanation is too long for a snackbar we may need to break it out into a separate onboarding dialog that's opened by tapping the snackbar.

The snackbar should be shown starting from a hardcoded activation date ~ 6 months after this ticket's released. The expiry date should be ~ 12 months after this ticket's released.

Subtask of #2221 (closed).