Makes lock screen long-press popup background fixed.

By design, the icon, text, and background colors should not be changed between light and dark theme. The current code only uses a fixed color for the icon and text but uses a theme-changing color for the background. In light theme, this means that the popup is displayed as dark icon and text over a dark background, manifesting as the attached bug.

Change-Id: Iba53a5742fd209bd8abbaf14eb57f2d9600f9b2c
Fix: 279068244
Test: manually verified that the bug no longer manifest in either light or dark theme, restarting System UI each time the system theme was switched. It's now always dark icon/text over light background, as per UX design.