Let contacts know if removable drive transport isn't supported
Android devices running API < 19 where the removable drive feature isn't supported should let their contacts know that the feature isn't supported. This could be done by not configuring the plugin on API < 19, so no transport properties will be sent.
A nicer but more complex solution would be to send different transport properties on API < 19 (supported = false), so the contact's device knows that we're running a Briar version that's aware of the transport, but our device doesn't support it, and thus the contact's UI can show an appropriate message.
Subtask of #1802.