Lock/login screens: 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
@teams/vdg (No screenshots since I think the above description should be adequate)