fix(binding/java): decode Java’s modified UTF-8 format

[G-XD]

Fix: #3194

[tisonkun]

Let's factor out this repeat conversion into jstring_to_string:

fn jstring_to_string(env: &mut JNIEnv, s: &JString) -> Result<String> {
    let res = unsafe { env.get_string_unchecked(&s)? };
    Ok(res.into())
}

I suspect we may have more thoughts on this conversion, so open to change.

[tisonkun]

Thank you!