location service: adapt to make use of the portal

GClue automatically calls the portal for us when sandboxed and in such case a NULL is returned for the client porxy because the calls goes through the portal.

Part of Initiatives#30

Marked as a draft until the geoclue changes lands upstream: landed, ideally we would get a tarball and the update will land in the SDK but as this only affects the flatpak bundle, we can keep geoclue bundled until that happens.