Adding customizable background color support for watch.

values-watch/themes.xml changes some Material themes, e.g.
Dialog inherits from Dark rather than Light. Hence I have moved
all of the theme modifications into
values-watch/themes_device_defaults.xml as they wouldn't be the same
for phones.

Tested: Manually on watch and on angler. Ran CTS host side theme
tests on cw-f-dev, all passed.

cts-tf > run cts -m CtsThemeHostTestCases

Bug: 31459804
Change-Id: I74a8502469cc76ac27a471c0bf18b22fd57f5418