Notification color overhaul.

Notifications now (based on a config) use the theme accent color instead of the app-provided color to colorize various elements within the notification (e.g. progress bar, actions).

Bug: 181048615
Test: exhausting visual inspection of as many notification combinations as the code led me to.
Change-Id: Ifbf07b27c65da9b17a0f0ecf6e1e6237126d015a