[SMS App] Basic plain-text MMS support

Summary

Desktop companion to https://invent.kde.org/kde/kdeconnect-android/merge_requests/78

Give desktop SMS app a basic understanding of the MMSes coming from Android:

• Show a fake body if we get an attachment we can't display (for now, any attachment)
• Display a fake contact header for multi-target messages since Android does not yet export multi-target address information
• Disable attempting to reply to multi-target messages

BUG: 398889

Test Plan

Before:

MMS messages were silently dropped, meaning:

• Group MMS conversations were not visible
• Single-target conversations with the most-recent message an MMS were not visible

After:

• Install https://invent.kde.org/kde/kdeconnect-android/merge_requests/78
• Multi-target conversations are displayed (kind of ugly, since they have no contact information
• 
• Single-target conversations which end with an MMS are displayed
• Plain-text MMS is displayed nicely
• 
• MMS attachments don't show
• 
  • MMS which are only an attachment with no body are displayed with a dummy body
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