lockscreen: Adjust to smaller screen sizes

This is another take on making the keypad fit.

• keep the keypad centered with margins as before
• drop the top margin on smaller displays

The upside is that we keep things as is for larger screens (and hence e.g. don't bottom align on 4K screens and laptops).

This patch contains the emergency button cleanup from @suraj_sloth (thanks!)

@suraj_sloth: Could you check if this works on your device.