Since the RNFetchBlob API doesn't support writing binary data directly
it creates a custom Writable which is doing the base64 encoding per
chunk.
This also fixes a problem with the S3 synchronization code using the
shim.fsDriver().writeBinaryFile().
Tested with AVD Android 10.0 target.
Signed-off-by: Jan Blunck <jblunck@users.noreply.github.com>