Themes: Switch themes when user changes

This patch grabs the theme config for the new user and then
creates a ThemeChangeRequest from that config.  This request is
then processed and applied for the new user.

Secondary users will not be able to change themes at this time.
This patch helps pave the way for that ability and provides a better
user experience when switching users.

Change-Id: I6d7e2ab312b8e3e5c099d1e9e2e62892bead10da