Stop using std::ffi::c_char as it doesn't exist on Android.

R=collinbaker@chromium.org

Bug: 1381552
Change-Id: I10bb95d4695cb3331fd2dc7b496c088a4e7259a2
Cq-Include-Trybots: luci.chromium.try:linux-rust-x64-dbg,linux-rust-x64-rel
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4007075


Commit-Queue: Łukasz Anforowicz <lukasza@chromium.org>
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Auto-Submit: danakj <danakj@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1067731}