Add "enhanced metrics collection" support

Add "enhanced metrics collection" support to the time zone detector service.

This commit doesn't make any actual changes to the metrics code, that
will be done separately. This just adds the infrastructure to support
the server flag to turn the behavior on.

Details:

"Enhanced metric collection" is for deployment to QA and other internal
users only. This initially means the actual time zone IDs will
optionally be recorded in the metrics collected by the device in
addition to the original "time zone ID ordinal" approach used for
general / public users.

Manual testing:

adb shell cmd time_zone_detector dump_metrics
adb shell cmd device_config put system_time enhanced_metrics_collection_enabled true
adb shell cmd time_zone_detector dump_metrics

Bug: 200279201
Test: atest services/tests/servicestests/src/com/android/server/timezonedetector/
Change-Id: Ibb480b748fb951f9781b3d6ccc9a14b682ff4a1a