Add test for MediaRouter2#getControllers

Test: atest mediaroutertest
Change-Id: I5ef3d3887281751cabe5d433a080e616a695e1aa

media/tests/MediaRouter/src/com/android/mediaroutertest/MediaRouter2Test.java

@@ -36,6 +36,7 @@ import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNotEquals;
 import static org.junit.Assert.assertNotNull;
+import static org.junit.Assert.assertSame;
 import static org.junit.Assert.assertTrue;
 import static org.testng.Assert.assertThrows;
 
@@ -693,6 +694,14 @@ public class MediaRouter2Test {
         assertFalse(systemController.isReleased());
     }
 
+    @Test
+    public void testControllers() {
+        List<RoutingController> controllers = mRouter2.getControllers();
+        assertNotNull(controllers);
+        assertFalse(controllers.isEmpty());
+        assertSame(mRouter2.getSystemController(), controllers.get(0));
+    }
+
     // Helper for getting routes easily
     static Map<String, MediaRoute2Info> createRouteMap(List<MediaRoute2Info> routes) {
         Map<String, MediaRoute2Info> routeMap = new HashMap<>();