Refactor DeviceDefault themes.

Adds colors_device_defaults.xml and defines the primary, primary dark, accent,
and secondary colors for the dark, light, and settings themes.

Also creates primary and primary dark values for the Material.Settings theme
and updates the themes to use those values.

Finally, fixes HelpUtils to properly get the value of colorAccent. Previously,
if colorAccent were defined to a hex value directly, it would crash.

BUG: 28625050
BUG: 29003416

Change-Id: Ia73b7368014c57c1676904d494cbb5169c8a836b