For #26519
This PR allows Fleet server to use Android with either fleetdm.com proxy
or locally. It also removes the Android feature flag from the backend.
The frontend changes and proxy API documentation will be in separate
PRs.
Updated contributor docs:
https://github.com/fleetdm/fleet/pull/29880/files
Integration tests are missing and tracked as a separate issue:
https://github.com/fleetdm/fleet/issues/27080
# Checklist for submitter
- [x] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
- [x] Added/updated automated tests
- [x] Manual QA for all new/changed functionality
