Fix bug removing semantic colors of CallStyle notification actions.

* When we changed the default color of emphasized actions to an accent color instead of the notification background color, I didn't split the two into different locals, and this meant the accent color was used to enforce the color contrast of the custom color.  This led to color desaturation in an attempt to meet contrast (when in dark mode) and a failure to ensure sufficient contrast (in light mode).
* A piece of this that applies specifically to colorized notifications is that we used the 'night mode' state as a shortcut for whether the background was light or dark.  This led to 1) different foreground colors for the same background color, depending on dark mode state, and 2) frequently a failure to meet contrast (and/or extreme desaturation) in one of those modes.
* Also adding different night-mode colors for CallStyle actions, to meet spec.

Test: validated sufficient contrast of default and various colorized background/button combos using the accessibility scanner.
Bug: 194935539
Change-Id: I3a69eadc89f1d9d98a816c44a36873c7615a177e