[lookandfeel] Fix wake existing screensaver mode with key presses

In commit 45e0a722 we changed the focus
on the textfield with the intention of fixing the cusor blinking.

This had the unfortunate side effect of breaking waking out of the
screensaver mode with just the keyboard.

That screensaver mode was bound to:
```
property bool blockUI: mainBlock.mainPasswordBox.text.length > 0
```
for that to change, we need to have focus.

Instead this patches cursor blinking more explicity.

Doing it only in the lock screen (as opposed to the toolkit) makes sense
as this only happens due to somewhat unusual usage inside the
lockscreen.

BUG: 431286