Integrate remote display routes into screen cast settings UI.

Merged remote display route selection into the existing wifi
display preference UI.  Moved the on/off toggle over to a menu item.

The preference page is now mainly implemented in terms of the
media router.  It only interacts with the display manager for the purpose
of pairing with new devices or making wifi display certification
controls available.

This means that the observed state is now completely consistent across
the system ui, settings, and applications that use the media router.

Bug: 11257292
Change-Id: I3705570812384fef4bfffeccaaccf7895d370d12