[SMS App] Make sms app not crash when conversation is selected with no devices connected

Summary: This patch fixes T10184 and stops the SMS app from crashing when a conversation is selected but no devices are connected. It also allows the SMS app to access the cached messages in the ConversationsDbusInterface so the app is still slightly useful even when the device is disconnected.

Test Plan:

• Open sms app
• Open a few conversations
• Disconnect phone (Force close app?)
• Re-open a conversation which was previously opened
  • Verify that the messages appear. It is possible to scroll up to view any older cached messages too!
• Open a conversation which was not opened previously
  • Verify that a single messages is shown (since this was the only one in cache, from populating the list of all conversations)
  • Verify that attempting to scroll this conversation does nothing, but also does not crash the app

Note: Opening the app with no phone connected will cause it to lose its handle on the deviceId, so it can't spawn a new Dbus interface, so it will remain blank and empty. Solving that is a project for another day.

Reviewers: KDE Connect

Reviewed By: KDE Connect

Subscribers: apol, nicolasfella, kdeconnect

Tags: KDE Connect

Maniphest Tasks: T10184

Differential Revision: https://phabricator.kde.org/D17634