Add CTS test for setBlockableSystem

Test: atest android.app.cts.NotificationChannelTest
Bug: 138401571
Change-Id: Ifabd7fb0d446a80f1f714a598d0aac6bdd9085bd
diff --git a/tests/app/src/android/app/cts/NotificationChannelTest.java b/tests/app/src/android/app/cts/NotificationChannelTest.java
index 5351ec8..19524ee 100644
--- a/tests/app/src/android/app/cts/NotificationChannelTest.java
+++ b/tests/app/src/android/app/cts/NotificationChannelTest.java
@@ -185,6 +185,13 @@
         assertEquals(false, channel.canBubble());
     }
 
+    public void testIsBlockableSystem() {
+        NotificationChannel channel =
+                new NotificationChannel("1", "one", IMPORTANCE_DEFAULT);
+        channel.setBlockableSystem(true);
+        assertTrue(channel.isBlockableSystem());
+    }
+
     public void testIsImportanceLockedByOEM() {
         NotificationChannel channel =
                 new NotificationChannel("1", "one", IMPORTANCE_DEFAULT);