Tidy up changes before API change

Extract non-API changes to make the API change commit smaller / more
focused. These changes are mostly naming, comment and some improved
validation. There's some toString() / parsing toString() changes and
associated tests for manual testing support used in later commits.

Test: atest services/tests/servicestests/src/com/android/server/timezonedetector/location/
Test: atest core/tests/coretests/src/android/service/timezone/
Bug: 236624675
Change-Id: I8f74e4bd0ad1a34b593afdf89b3e4840420a5d10