Improve text contrast ratio in Glanceable Hub empty state

Updated empty state text color to use onPrimary instead of primary color
to improve readability and meet accessibility standards. This addresses
contrast ratio issues in both light and dark themes where text was
previously below the recommended 4.5:1 ratio

Bug: 378418098
Test: Device tested
Flag: NONE Accessibility fix
Change-Id: I391e2f02c6fe3db24576b906e45ce1551e491d28