Update tint for minimize button

Attribute used for the tint color does not seem to exist in WMShell.
Maximize button uses @color/decor_button_dark_color. Using the same for
the minimize button. They probably should look the same.

Test: run WMShell unit tests from studio
Bug: 242094334
Change-Id: I9d852c0d8e1a63438f89f1bc813725e1c7caf1c7