Allow users to change persisted admin password option

Hi,

#17136 (closed) added the possibility to save settings in the welcome screen.

However, it seems it only save admin password setting the first time it is selected. I found no way to persistently rewrite this setting. So far, all the other options I tested (language options, connection mode, unsafe browser activation) were successfully rewrote with the last selected option, only the admin password is not, and have to be disabled every time (or welcome screen persistence fully disabled).

Originally reported by u/weMetinDecember on reddit: https://www.reddit.com/r/tails/comments/jtif6f/tails_forces_administrator_password_even_when/

cc @segfault @sajolida.