Fix SntpClient to give accurate time info.

Old calc was off by 2x and was affected by the user-settable system clock.  The error
came because it was calculating the offset between our clock and the NTP clock and the algebra
had two factors of the offset instead of the desired 1.

bug:2600010
Change-Id: I0856091d32b50e6909e4889fb98df819e0aeabbe