Refactor Settings theme colors.

Introduces stock colors (primary_color, primary_dark_color, etc) to allow
overriding the colors via resource overlays.

Creates new colorSecondary attribute and updates switches to use theme
attributes for foreground/background colors rather than hardcoded values.

Some other mild color cleanup to make things more straightforward.

BUG: 28760785
Change-Id: I4deba962357217bdb2e08c08e8f83b9b57a683d4