For #26218
- Added `GET /api/_version_/fleet/android_enterprise/signup_sse`
endpoint and tests
- Fixed up handling of Android status reports with a deleted device. We
don't actually expect this to happen in production since the proxy
should delete the pubSub connection when the enterprise is deleted.
# Checklist for submitter
- [x] Added/updated automated tests
- [x] Manual QA for all new/changed functionality