Separates out view_manager (server) into a two gn targets

There is now view_manager_lib which view_manager depends upon. Non app
tests that want view_manager now link in view_manager_lib. This lets
me get rid of all the EXPORT stuff as that isn't something we really
want.

R=erg@chromium.org

Review URL: https://codereview.chromium.org/714763002