[native] replace folly::Optional with std::optional

Summary:
The point is to avoid adding folly dependency on web.

Emscripten C++17 compatibility will be handled in subsequent diff.

Depends on D8472

Test Plan:

1. Log out
2. Call getOlmPersistAccountData() -> should return nullopt`
3. Log in
4. Call getOlmPersistAccountData()` -> should return string
5. Tested on both iOS and Android.

Reviewers: michal, tomek, jon, atul, marcin

Reviewed By: michal, tomek, marcin

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8487