For #26218
Basic Android MDM on/off backend functionality. Manually tested.
The following env vars must be set:
```
FLEET_DEV_ANDROID_ENABLED=1
FLEET_DEV_ANDROID_SERVICE_CREDENTIALS=$(cat credentials.json)
FLEET_DEV_ANDROID_PUBSUB_TOPIC=projects/your-project/topics/your-topic
```
I picked https://github.com/go-json-experiment/json as the JSON library,
which seems like the safest option.
- will become json/v2 at some point
- currently used in production by other companies, like Tailscale
- well-maintained
- Some context here: https://github.com/fleetdm/fleet/issues/25512
Plan for next work:
- refactoring from 1st PR
- add pubsub with device enroll -> spec proxy for fleetdm.com
- come back to this sub-task to add tests and finish TODOs
# Checklist for submitter
- [x] Added/updated automated tests
- [x] Manual QA for all new/changed functionality
fc33df1981