Summary:
Apparently certain Android NDK does not support numeric conversion functions of
`basic_string`. Use folly/Conv instead.

Changes generated by `%s/std::to_string/to<std::string>/g`.

In addition, Android SDK's treatment of `long double` is inconsistent. Avoid
that in the tests for now.

Reviewed By: yfeldblum

Differential Revision: D9196009

fbshipit-source-id: 54a32c426a4beb6a0e2d7f95d6b30bf6d7cc67cf