Fix text color in multiuser menu when option is disabled

In the current version, the color is hardcoded and is not dark theme aware. Therefore when the menu option is disabled and the dark mode is enabled, the text is not visible. The change in this CL retrieves text color from the context's theme.

Bug: 299692354
Test: manual, opened the menu in both dark and light theme with "Disallow remove user" restriction, ensured text is readable
Change-Id: I087b06526af03bd147db5b40c2f6a82d90742898