[SMSApp] Support plain-text MMS

Summary

Not having support for MMS caused some minor problems, like in https://bugs.kde.org/show_bug.cgi?id=398889 . This patch adds basic MMS support for plain-text MMS, including multi-target messages.

Android companion to https://invent.kde.org/kde/kdeconnect-kde/merge_requests/97

Currently there are several rough areas:

• Multi-target messages do not have the full list of recipients (I am planning to work on this in another patch, because this one is already quite large enough)
• Parsing MMS is significantly slower than parsing SMS. This makes sense, since we need to make significantly many more content:// calls for MMS. The only solution I can think of here is to add the ability to request a range of messages, which I need to do anyway, but which should not be part of this patch.
• The desktop app is totally busted with regard to multi-target MMS, but that will also be fixed in another MR

BUG: 398889

Test Plan

Before:

Open SMS app on desktop, scroll through conversations, notice:

• Any single-target message which had the most-recent message as an MMS does not appear
• Any multi-target MMS conversations do not appear

After:

Open SMS app on desktop, notice:

• Conversations which have an MMS as their most-recent message appear
• MMS which consisted of only text are rendered correctly
• Multi-target conversations are shown (though pretty busted, as said before. Do not attempt to reply to one!)