Add metadata for mobile devices using Phosh

[baarkerlounger]

This will enable WatchMate to show up as Mobile friendly on the default app screen in Phosh and in the PureOS app store.

[azymohliad]

Nice! I didn't know about those Purism-specific keys (I thought it's primarily an issue with Flatpak not supporting standard hardware metadata).

Thank you!

[azymohliad]

I want to make one very minor addition this weekend (bundle icons), and then release it together