[native] Update panelSecondaryForeground to shadesBlack80

Summary:
In the previous diff, I was implementing a design where the walletIdentifierTile (panel foreground) had a tile inside it. I figured that inner tile should logically use the "panel secondary foreground" color.

However, panelSecondaryForeground was currently set to #333333, which does not appear in our design system, and did not match the designs for the inner tile.

This diff updates panelSecondaryForeground to match the designs for the inner tile, and makes sure it's now using a color from the design system. The only other place that is impacted is the calendar tab.

Test Plan:

	before	after
registration
calendar

Reviewers: ted

Reviewed By: ted

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D7887