The module analytic_user_function allows to define a specific product
to use, when creating timesheet activities for a specific employee
with a specific contract
Nevertheless, the product was not set accordingly to this feature
for the first timesheet activity, because,
when initilialing the timesheet line,
the on_change_account_id, which returns
the product to use according to the user and contract, was called
without passing the user.
Besides, by default, on_change_account_id does not have a user_id parameter,
this parameter is added by the module analytic_user_function, overriding
this onchange, and adding a new user_id parameter (which is not a good pratice).
We therefore use multi_on_change_account_id, which allow to pass the user_id,
within the context
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