Switch from Jackson to plain JSONObjects for exporting message history

[robinlinden]

Exporting the message history using Jackson on Android 4.4 leads to the app crashing with a NoClassDefFoundError.

See: https://github.com/FasterXML/jackson-databind/issues/2807

I compared a before and after export using diff <(jq . jackson.json) <(jq . jsonobject.json) and this results in no change other than a locationSaved top-level field disappearing.

[robinlinden]

After this, it is very little work to change up the format if we want to e.g. not include the public key in every message entry (which we probably don't,) but this change is set up to alter the output as little as possible, including preserving the pretty-printing whitespace. :P

[theAkito]

As long as the root object is this simple to manually construct & we get rid of a superfluous dependency, it's all worth it. It's actually an improvement, despite the root reason being an error in the library.