fix: always use display name from correct backend

fix: always use display name from correct backend

Overwrite the display name after the account is initialized
when using an instacne of IGetDisplayNameBackend.

Before when using a variation of user_oidc and registering
a Backend.php implementing IGetDisplayNameBackend
the personal setting page shows 'uid'.

The UserManager/AccountManager seems not to use consistently
the correct backend.

The correct backend is used in this sequence:

server/lib/private/TemplateLayout.php

          $userDisplayName = \OC_User::getDisplayName();
          $this->assign(user_displayname, $userDisplayName);

In the settings page, it definitely not calls the registered backend,
but seems to fall back to default Backend and shows (usually) uid
or a value from the standard account property table.

Signed-off-by: Max <max@nextcloud.com>