Better expose text color for single button panel button

When we did the original expose pass for 13.0, we lumped too many
values into the same custom color. Let's give this button text color its
own value so that it can be themed individually from other instances of
text_color_white.

Change-Id: I4b76633eee6ebcb0f560ff527f081fa80c16850a
Ticket: CYNGNOS-2179
(cherry picked from commit 32db916c)