TwilightService: Ignore (0,0) coordinate location updates

Location providers may erroneously return (0.0, 0.0) when they
fail to determine the device's location. These location updates
can be safely ignored since the chance of a user actually being
at these coordinates is quite low.

Bug: 31009261
Test: added TwilightServiceTest; both tests pass

Change-Id: Ie9d7d10f81ade9b0be10f98ed74978539efaa658