Lock/login themes: stop adjusting shadows based on color scheme

Right now we change the intensity of shadows based on the color scheme's
text color, but with a property called "lightBackground". This indicates
that we wanted to do it based on the background color itself, but did
not or could not, so settled on using the text color as a proxy. However
this is wrong; there is no relationship between the active color
scheme's text color and the color of the background image.

Even if we could or did fix that, it would probably still be wrong,
because using lighter shadows on a genuinely light background would
result in worse contrast and readability, especially if the light
background was very visually busy.

For these reasons, in most other places, we have since settled on the
mantra of "shadows should be black", and this commit makes the lock and
login screens follow that convention as well.

BUG: 449985
FIXED-IN: 5.24.2