Hide the icon if there is no media session or album art.

- Before this CL, the output switch panel will show a default
  icon if there is no media session or album art.

  This CL will hide the icon if there is no media session
  or album art.
- Add test case

Bug: 161495909
Test: make -j42 RunSettingsRoboTests
Change-Id: I5f80158b12f89c8499fb97d0b203ebeffefbc18b
Merged-In: I5f80158b12f89c8499fb97d0b203ebeffefbc18b
(cherry picked from commit 30805af9)