Fix share button color in dark mode for clipboard edit

Use the textColorPrimary color for the share button color so that
the (borderless) share button icon shows up properly in dark mode.

Bug: 221508156
Fix: 221508156
Test: manual (visual change; checked that button is visible in
both light and dark modes)

Change-Id: I76f9e2acd883b9d6e00bd37b2c0ba0f7d242502e