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 30805af9de)

res/layout/panel_layout.xml

@@ -36,6 +36,7 @@
             android:visibility="gone">
 
             <LinearLayout
+                android:id="@+id/title_group"
                 android:layout_width="wrap_content"
                 android:layout_height="wrap_content"
                 android:orientation="horizontal"