BugFix: Density-based coarsening: Set the accuracy to the approximate edge of the S2 cell.

Currently the accuracy of coarsened locations is set to the size of the grid. With the new algorithm, the coarsening will depend on the population density. The accuracy (which is a proxy for the size of the blue disc in maps apps) should scale with the size of the coarsened region.

Because the S2 cell library does not have a function to calculate the edge length, we use the average area to estimate the edge. This approximation assumes that all S2 cells are squares.

Tests:
- atest FrameworksMockingServicesTests:LocationFudgerTest

Change-Id: If172fff9cf1d74b4f86f0e9832d946b35d468362
Test: manual atest on Pixel 7 pro (see above)
Bug: 376198890
Flag: android.location.flags.density_based_coarse_locations