[native] move olm/olm.h include to proper file
Summary:
We do not need this header Tools.h, it's not used here, but it's not included in Session.h where it's needed.
This cause 2 issues:
- Required using #ifndef EMSCRIPTEN, because Tools.h is used on web, and we do not want to add olm.h to bundle.
- Breaking our C++ coding standards: src.
Test Plan: Build iOS, Android, WASM.
Reviewers: michal, tomek, jon, marcin
Reviewed By: tomek
Subscribers: ashoat
Differential Revision: https://phab.comm.dev/D8672