[FIX] mail: fix performance issue for activity view

Fun fact: for the subscription model in odoo production database, the
get_activity_data rpc takes 36s, and transfer 420mb (non gzipped) of
data (when looking at all subscriptions).

The main reason for that is that the get_activity_data method create a
domain with all the res_ids (so, if you have 20k subscriptions, the
domain will look like (res_id, in, [...20k ids]).  This domain is given
to read group, and the resulting groups (each with a domain key which
includes those res_ids) will be sent back to the web client.

As far as I can tell, the domain key is not used by the web client, so
we send (number of groups)*(number of records) of useless data.

This commit improves the situation in two different ways:
- we only generate the domain if there is a domain at the beginning.
  This should greatly improve the performance when there is no current
  domain
- we do not transfer the data to the web client

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