Ship preinstalled Flatpaks

Title. A lot of the apps we ship OOTB can just be Flatpaks we install.

However, this becomes challenging because we have some goals I'd like to maintain:

• users able to uninstall these apps for themselves, but not other users
• factory resets should work, and the apps should be re-installed
• ISO should have a try mode w/ the apps installed already

Some other questions come up:

• how to integrate downloading Flatpaks w/ buildstream
• how do I ship/where do I store the flatpaks?
• how to reconcile those goals above
• how do I handle OS updates adding/removing pre-installed apps?