Mobile: Fixes #3601: Fix uploading resource files to S3 with RN (#4127)

When the mobile application tries to synchronize a note with attached
resource files, e.g. from the notebook "Welcome! (Mobile)", readFile()
is called to return a Buffer although that isn't implemented in
FsDriverRN.readFile(). This is changing the code to return base64
encoded content from react-native and turn it into a Buffer before the S3
putObject() API call is used to create the remote object.

Tested with AVD Android 10.0 target.

Signed-off-by: Jan Blunck <jblunck@users.noreply.github.com>