[lib] limit report size to 200MB

Summary:
Bigger reports will crash app anyway so avoid adding them to store.

I checked this with sizeof(std::string::value_type) in both iOS and Android, and (with no surprises) one char size is 1 byte. So assuming we can expect size to be approx string length without std::string overhead which was ~90000B with larger reports.

Test Plan: Generate large reports and verify that is not added

Reviewers: michal, tomek

Reviewed By: tomek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D8040