session: Use the default OSK instead of Virtboard

Needs librem5-base!41 (merged)

This will complete the story for letting a user install squeekboard and test it out.

To test, install squeekboard and librem5-base-defaults (after the above MR is in):

apt-get install squeekboard librem5-base-defaults

and then start the session anew, e.g.

systemctl restart phosh