When changing password, a PolicyKit prompt appears and ask for the **new** password

Step to reproduce:

0. Assumes that you're originally on the Passcode mode.
1. Go to Settings -> Security & Privacy -> Locking and unlocking -> Lock Security
2. Select "Passphrase". Enter old & new password. Press "Set".
   • Expected result: the password is changed successfully with no further prompting.
   • Actual result: PolicyKit prompt appears. The prompt said "Authentication is required to change your own user password". However, the new password has to be typed instead. This is very confusing.

Still, I believe it's bad UX to have additional password prompt(s) appear, old or new password. I've already raised the potential for the additional prompt problem in !20 (comment 1689422011) , yet the code is merged without addressing my concern.

So, for now I think it's better to bring back internal PolicyKit agent and remove Lomiri's agent, to maintain the known-good UX. There might be a way for both to co-exists, or there might be some other solution to maintain the UX. But as-is, we can't ship Ubuntu Touch Focal OTA-4 in this arguably broken state.