Fix Rust string parceling

The NDK `writeString` API accepts a UTF-8 string with embedded NULLs,
not just C-style null-terminated strings. This change allows parceling
of Rust strings directly, without requiring conversion to `CString`
first.

Test: atest libbinder_rs-internal_test
Change-Id: I0c5f9a116a17cc3f48622ed3e8f32b8cbe8af891