Show the output switcher when no media is playing

- Support output switcher for system routing.
- Add an new string to indicate the device that
  audio will output to.

Bug: 284227163
Test: device/host atest
 atest MediaOutputPreferenceControllerTest
 atest AudioOutputSwitchPreferenceControllerTest
 atest MediaOutputIndicatorSliceTest

Change-Id: I94bcf84e7e93b3e4f5db1d95d5380a54a3e0c460
Signed-off-by: Jasmine Cha <chajasmine@google.com>