CountryDetector: Enable detector class override for automotive

The detector class to be used by the CountryDetectorService can be set
by overlaying config_customCountryDetector in the device config.

Test: atest CountryDetectorServiceTest
Bug: 141626568
Merged-In: I158abd6397e3102d13e431ba043e8ce45061b64a
Change-Id: I158abd6397e3102d13e431ba043e8ce45061b64a
(cherry picked from commit 34147601d85517847722d12ab8fad2be48cd0d31)