Incorrect time used in some NTP server responses

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