In plugins/fs/src/android.rs resolve_content_uri, a null `fd` from
getFileDescriptor (openAssetFileDescriptor returning null, e.g. for
virtual files) hit unimplemented!() and panicked inside the command handler.
It now returns an Unsupported I/O error. Also switches the two
io::Error::new(Other, ..) calls to io::Error::other, which clippy flags on
the Android target. Checked with cargo clippy --target aarch64-linux-android.
