Save configuration when creating resources for new user

Summary:
Akonadi calls writeConfig DBus method to save the configuration for newly
created resources, both via firstrun and SpecialCollections mechanisms.
This method is non-existent for all of the resources (it was deprecated in
KConfig, and apparently is not exported to DBus), method save should
be used instead.

This is related to issues raised in D27905: [resources/maildir] Reload configuraton on configuration change: settings provided in firstrun did
not override default settings for the resource.

CCBUG: 345211
CCBUG: 352502

Test Plan:

1. Check qdbus org.freedesktop.Akonadi.Resource.akonadi_maildir_resource_0 /Settings
2. There is no writeConfig method, but there is save method

Reviewers: dvratil

Reviewed By: dvratil

Subscribers: kde-pim

Tags: KDE PIM

Differential Revision: https://phabricator.kde.org/D28523

(cherry picked from commit 4447e8894d4f3b4c08849956f33aa7156caa30a4)