set-datetime: Make timesyncd behave in the face of broken time

The set-datetime runonce would set the time correctly for the first boot, but
subsequent boots would fail to persist the change due to broken behavior on the
part of systemd-timesyncd.

We now stop systemd-timesyncd, forcibly set its clock timestamp file, and then
reboot to avoid this broken behavior.

Change-Id: Ib5950b3ce917724e88aeff19e408d05f549f3882