Per RFC 4330, a NTP server response should be discarded when:
- the stratum is 0 (unspecified), or
- the leap indicator is 3 (unsync'ed), or
- the mode is not 4 (server) / 5 (broadcast), or
- the transmitted time is 0.
Update SntpClient so that such responses would be discarded.
Additionally:
- make some variables suitably "final"
- enable logging
- add alternate requestTime() for testing
- add some miniscule test coverage
Cherry-picked from Jerry Wong's
https://partner-android-review.googlesource.com/#/c/460074
Bug: 24719581
Change-Id: Id11a79a6e53ce95500ed4b4d691a29c260666f6c
32d52f344a