Add flags for population density provider and density-based coarse locations.

This is in preparation for the Android B workstream about density-based coarse locations (details in go/density-based-coarse-locations). TLDR: Down the road, approximate locations might get coarser in low-population areas. For this, we will add PopulationDensityProvider in AOSP, that GMSCore (or OEMs) will implement.

We need two flags:

1) We'll need to add a IPopulationDensityProvider in AOSP, that GMSCore or OEMs can implement. There is one flag for this, so we can completely disable this set of changes.

2) Later on, we'll need to actually call the IDensityProvider in LocationFudger. This is a separate flag.

Change-Id: I9feab6b73893a1ba973d7d33f82e265cacb2e77f
Test: None
Bug: 376198890
Flag: EXEMPT only adding a new flag definition