Fix issue with partial UTF-8 string (#6317)

Summary:
It will cause JNI exception if we don't pass in UTF-8 string.

Alternative 1 (this): wait until we have complete UTF-8 tokens.
Alternative 2 (?): Fix this from runner layer
Alternative 3 (no): Change the API to use uint8_t array, but if we want to display on app in real time, this is still an issue.

Pull Request resolved: https://github.com/pytorch/executorch/pull/6317

Reviewed By: Riandy

Differential Revision: D64580932

Pulled By: kirklandsign

fbshipit-source-id: 341ea906097707fae0f97d32dea974ad44425083