Let apps receive location updates in the background

This is a rebase of !7 (closed) by @mardy onto the main branch.

Contributes to: https://github.com/ubports/ubuntu-touch/issues/1067

Other related branches:

• https://github.com/ubports/qtubuntu-sensors/pull/6 (not in focal)
• https://gitlab.com/ubports/core/location-service/-/merge_requests/12 location-service!20