checkbox-touch: make remember_password use new pyref approach

This patch makes logic responsible for calling remember_password on the python
side to use new PythonObjectRef instead of 'handle' approach that's missing
now.

Fixes: https://bugs.launchpad.net/checkbox-touch/+bug/1468531
Signed-off-by: Maciej Kisielewski <maciej.kisielewski@canonical.com>