Does a dedicated transport for a troops unit retain objective secured after the troops disembark?
It does, and you can. You don't even have to start the Strike team in the devilfish, so you could begin the game holding two objectives. A transport bought separately (generally most other transports are Fast Attack) will not have Objective Secured unless it is "Troops" or the mission says it's type has ObSec. 

It's worth noting that if you have Pathfinders in a Devilfish and Firewarriors in a second, only one of them will have Objective Secured. The only really confusing situation is if both units disembark and then switch vehicles... Units in a transport do not confer (or revoke) ObSec status to the vehicle.