Add location-based tz detection in the background

Add support for running location-based tz detection in the background.

This is intended for use during QA / internal testing to understand the
behavior without affecting the tester's actual device time zone. For
example, this could help prove that the feature is broken with certain
flags without inconveniencing the testers too much: they can turn off
"Use location to set time zone" and we can still get useful feedback.

Test: atest services/tests/servicestests/src/com/android/server/timezonedetector/
Bug: 200279201
Change-Id: I63a3cd02293a1c2322abb669b9804ac560858ba1