Disable entry point of output switcher

It adds a minimum value if it shows only one available
cast device in the output switcher. Because users can only
change the volume slider or stop control in the output switcher.
It's too hidden to have the user stop cast in the UI.

- This CL will disable the entry point of the output switcher
  if there is only one available cast device in the list.
- Update test cases.

Bug: 163095048
Test: make -j42 RunSettingsRoboTests
Change-Id: I8906878e1ba769d6940041f17d83b5de6b2a32c0