[FIX] google_calendar: exclusions not propagating (from OE)

Activate Google Synchronization, create on OE a recurrent event,
synchronize the calendar, then delete an event of the recursion on GC,
sync again on OE.

The event will be deleted from GC but not from OE after sync.

This appens because of this "rewrite" rule
https://github.com/odoo/odoo/blob/12.0/addons/calendar/models/calendar.py#L918


that occur on event creation from OE, altering the event parameters when
is marked "allday".

When an "allday" event is deleted from GC the unlink is triggered in OE with the
default time "00:00:00". During the creation of the exclusion '
_inverse_dates' will be called altering start and stop datetime but not
recurrent_id_date, so the new record will not match the event generating
the recursion and the exclusion will not occur. The problem require
particular carefulness because when a recurrent event is fetched from
google the '_inverse_dates' is not called, so in that case the default
time is fine.

opw-2060526

closes odoo/odoo#39653

X-original-commit: a468464c
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>