What is a "roving" backdoor?
I understand the general concept of a backdoor.
Emphasis on keyword qualifier: "roving"...
In the context you provide, it has to do with a backdoor that changes or moves over time. There might be a backdoor on port 8000, then it switches to port 8001, etc. The idea is that once a backdoor is discovered, the hacker must re-discover it to use it again.
See also the concept of "port knocking" in real life, where the backdoor only opens once you trigger it with a sequence of steps.
In surveillance I believe the concept of 'roving' is down to the court order placed on an individual.
For example; If an authority, be it the police, intelligence or security services is issued a roving surveillance order by a court to tap your phone, then they tap your phone, but then you dispose or make inactive that phone, then the order will allow tapping of further devices that belong to you, without having to re-apply for a further court order. So the authority can continue monitoring your communications.