[Lock screen] Make clearPassword() do what it says and then use it

Right now the clearPassword() signal does not actually clear the
password; it only selects all text. This is a violation of its name
as well as being pointless since there's no longer a way to see the
unmasked text, which means you always need to re-enter the whole
password anyway.

BUG: 412252
FIXED-IN: 5.20