Mobile: Fixes #4268: Fix "Not implemented" error when downloading resources with S3 sync target (#4279)

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>