main/postmarketos-ui-weston: switch to tinydm

Tested and works in QEMU.

I think it makes sense to use TinyDM for Weston as Weston primarily is used for testing purposes and not actual usage, so the lack of prompting for a password is ideal if anything. Besides, I don't think LightDM was configured to prompt for that in this package anyway.

Closes https://gitlab.com/postmarketOS/pmaports/-/issues/871 as all UI:s in the list are converted now.