[nemo-qml-plugin-systemsettings] Refactor location settings. Contributes to JB#47257

Adjusted for better maintainability.
- Api addition: pendingAgreements tells which location services
are missing acceptance on currently set location mode
- Some code moved around for grouping both deprecated parts and
location services together
- Yandex locator enabled setting removed, was copy-paste detail from
mls which has both offline and online variants.
- agps_providers on settings file didn't appear used by anything. Removed.
- Removed writing deprecated keys, only need to read them (if even that).
- Simplified lots of things

Author: pvuorela