Once #78113 gets merged, we should review the usability & improve if necessary.

Ideas:

• don't create the custom devices config file as a side-effect when running the AppContext
• maybe instead create it once when the custom devices feature is enabled
• Give the user the path to the config file when the feature is enabled, so the user knows what to edit
• consider adding a flutter custom-devices subcommand for listing the currently configured devices, checking if the config is valid and maybe for adding/editing/removing devices
• Is non-interactive usage (by scripts) a thing we need to look out for? If yes, maybe we can improve something there as well?