Add PopulationDensityProvider

This change adds the concept of population density providers in AOSP. It allows the framework to query the population density at a given location. Internally, this is backed by a new type of Provider, following the current conventions in the location package. There is no app-facing change, only the LocationManagerService will access this provider (see other CLs in the topic).

Because the provider will be implemented by Play Services (or OEM implementations), we add a proxy mechanism identical to the current ProxyLocationProvider, which reads the config.xml and instantiates the correct provider.

The core of the change is IPopulationDensityProvider.aidl.

In details:
- We add a new IPopulationDensityProvider IDL with its associated implementation PopulationDensityProviderBase.
- We add the IDL for two callback type for the S2CellId and the default coarsening level.
- We change the LocationManagerService to instantiate a ProxyPopulationDensityProvider.
- This ProxyPopulationDensityProvider creates the correct provider given the XML config.

Tests:
- atest CtsLocationNoneTestCases:PopulationDensityProviderBaseTest
- atest FrameworksMockingServicesTests:LocationManagerServiceTest

NB: I also did a manual test by implementing a fake provider and querying it, this works on Pixel 7 pro, see linked commits with topic "population-density-provider-test-dns"

Test: manual atest on Pixel 7 pro (see above)
Bug: 376198890
Flag: android.location.flags.population_density_provider
Change-Id: I90663d478200c734abbf5f442dfb1bb8bb79a875