TimeDetector improvements

TimeDetector improvements for production code and tests. Many of these
are based on feedback given for TimeZoneDetector and to improve
consistency across the two.

1) Add a builder for PhoneTimeSuggestion, making it immutable (except
for debug info that doesn't affect behavior).
2) Improve debug logging.
3) Improve comments / annotations.

Test: atest android.app.timedetector
Test: atest com.android.server.timedetector
Bug: 140712361
Change-Id: Ibd0d357c536d512ffe4995c4e6560e483a260ae5