Use colorSecondary for darker bg color.

Used for widget headers and a few other places.

In order to support pre-API 25, an app-local version of colorSecondary
is created and used throughout the app. A new WidgetContainerTheme
sets colorSecondary to a canned value. On API 25+, colorSecondary
is set to the platform value.

Change-Id: Ie0dd66333052b7346eea5d82d1c88417c6a2cd00
Fixes: 28625102