do not pre-install messenger/default apps as **system apps**

The pre-installed apps (signal/telegram, etc.) should not be installed as system apps, to allow uninstalling when using an alternative.

[One alternative, for example, is to use an Email-chat spec compatible messenger. The upcoming frontend update of https://delta.chat for example is based on the signal app and may then also be usefull as SMS app.]

Also see "Can Signal and Telegram be removed or disabled?" https://community.e.foundation/t/can-signal-and-telegram-be-removed-or-disabled/605

Or the general "Howto remove Default apps?"https://community.e.foundation/t/howto-remove-default-apps/1227