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)

tests/robotests/src/com/android/settings/panel/MediaOutputPanelTest.java

@@ -317,4 +317,20 @@ public class MediaOutputPanelTest {
     public void getViewType_checkType() {
         assertThat(mPanel.getViewType()).isEqualTo(PanelContent.VIEW_TYPE_SLIDER_LARGE_ICON);
     }
+
+    @Test
+    public void getIcon_mediaControllerIsNull_returnNull() {
+        mMediaControllers.clear();
+        mPanel.onStart();
+
+        assertThat(mPanel.getIcon()).isNull();
+    }
+
+    @Test
+    public void getIcon_mediaMetadataIsNull_returnNull() {
+        mPanel.onStart();
+        when(mMediaController.getMetadata()).thenReturn(null);
+
+        assertThat(mPanel.getIcon()).isNull();
+    }
 }