agent-manager: return all secrets if we don't need to ask agents

When the connection already has all needed secrets,
_con_get_try_complete_early() returns `existing_secrets` without
asking to agents; `existing_secrets` is initialized with system
secrets.

In NMSettingsConnection, get_secrets_done_cb() strips all secrets from
the connection and then merges the secrets returned by
nm_agent_manager_get_secrets(). Since nm_agent_manager_get_secrets()
returns only system secrets, the result is that agent owned secrets
are not asked to agents and are removed from the connection.

Fix this by returning all secrets from the agent manager.

https://gitlab.gnome.org/GNOME/network-manager-applet/-/issues/157
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