Hide the icon if didn't have media session or album art

- This CL before, output switcher panel will show default
  icon if there are no media session or album art.

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

Bug: 161495909
Test: make -j42 RunSettingsRoboTests
Change-Id: I5f80158b12f89c8499fb97d0b203ebeffefbc18b

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

@@ -318,4 +318,20 @@ 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();
+    }
 }