[FIX] google_calendar: fix allday timezone synchronisation

When syncrhonizing an allday event while being in a negative timezone
e.g. Dec 22 in US/Eastner (-5:00)

The event was synchronized the day before
e.g. Dec 21

because `context_timestamp` of `start date-time-` was used,
and then the time was removed to have the date of the all day event
e.g. 2016-12-22 00:00:00 converted to 2016-12-21 19:00:00 and then the time
removed 2016-12-21.

Instead of using the `start_datetime`, we now use the start
date which stores only the date, without timezone information
(and this is what you would like in the case of an all day event).

opw-697202