[FIX] hr_holidays: correct the timezone for Public Time Off

When we create a public time off,
we save the start date and the end date in utc.
Unfortunately, to calculate the utc, we don't use
the calendar timezone linked to these public time off.

Issue:
For example, with the "timesheet_grid" module,
the timesheets created for these time offs
will not be correct for employees who have a calendar
in a different timezone than the user who creates the public time off.

Solution:
Convert the dates received in the calendar timezone.

opw-3302925

closes odoo/odoo#121171

Signed-off-by: Yannick Tivisse (yti) <yti@odoo.com>