Make some notification related slices public.

Only PreferenceControllers with isPublicSlice() set to true are exposed
to other apps. Others will be Settings only.

Bug: 141088937
Test: robotests
Change-Id: I6e7b9d45084ea9944143e606bfac56b2cae3e8a3

tests/robotests/src/com/android/settings/notification/MediaVolumePreferenceControllerTest.java

@@ -63,4 +63,9 @@ public class MediaVolumePreferenceControllerTest {
                 mContext);
         assertThat(controller.isSliceable()).isTrue();
     }
+
+    @Test
+    public void isPublicSlice_returnTrue() {
+        assertThat(mController.isPublicSlice()).isTrue();
+    }
 }