PLAT-11641 handle json serialisation errors and remove payloads

[richardelms]

Goal

Error handling

Sometimes, while flushing the session and event cache, a serialisation error can occur.

This PR improves the handling of those errors. The new setup is as follows:

• if an error occurs while trying to serialise the json into a dictionary then we silently remove the offending file, as it is most likely caused by an unpreventable IO error.
• if an error occurs while trying to create an event or session from the dictionary then we remove the file. At a later date we will implement reporting this issue internally.

File writing

It could be possible the the method File.writeAllText is buffered under the hood. So we are now using a file stream to write data and using the method FileStream.Flush to force it to write all at once.

Testing

Covered by existing tests

[bugsnagbot]

Build size impact

Format	Size impact of Bugsnag (kB)
IL2CPP APK	2348.56
IL2CPP IPA	1348.39

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