Avoid transmitting MMS messages when prohibited by roaming preferences

Monitors the cellular status and roaming preferences to:

 * Require manually triggered downloads for incoming MMS while roaming
 * Fail the automatic attempt at sending MMS while roaming

unless the preferences specify that data is always allowed when roaming. If the user manually initiates a download or a retried send, that will be allowed. If the policy is to never use data when roaming (as opposed to 'Always ask'), it will still be blocked by ofono.

Some of this behavior is a temporary measure until we're able to present a warning about roaming in the UI, but at least the download logic will be permanent.