main/postmarketos-ui-plasma-mobile: re-enable auto-login

Effectively revert !1382 (merged) for Plasma Mobile. As discussed on Matrix, upstream doesn't support non-autologin and they even enabled a lockscreen on boot recently. As much as I dislike a hardcoded username like this personally, we shouldn't deviate from upstream too much.

Besides, the current approach appears to be broken at least for Plasma Mobile as seen in #789 (closed), which this resolves as well.