Lockscreen: Move the unlock screen to the bottom and small UI tweaks

This makes the unlock screen stick to the bottom of the screen, with the same bottom margin as the screen with the time. This also changes the "submit" button to "unlock"

@tobias.bernard asked me to add also some small tweaks

• Make the "submit" btn more rounded and align it with the numbers of the keypad

Fixes: https://source.puri.sm/Librem5/phosh/issues/235