main/postmarketos-ui-plasma-mobile: install pre-configured kdewallet

The dialog to set a kwallet password is currently broken and unusable if you try to set a password. To prevent this, just install a pre-configured wallet without a password. Not a great solution, but better than having the functionality be broken for now.

Proper fix would be upstream fixing the virtual keyboard so users can enter values in the right boxes. Once that is fixed, we can (and should) revert this

This patch was originally added as !2079 (merged) to 21.03, instead of to master first and then getting cherry-picked to edge as the process should be. For consistency, put it in edge too.

@PureTryOut: any reason not to merge this to edge?

CI is expected to fail for armv7 due to this, let's merge anyway if it passes for other arches (e.g. by force-push with MR-ID added and [ci:skip-build] in the comment after it already built in CI).