use an observer for the default sound

matches other settings use, and also improves testability of buzzbeepblink

Change-Id: I9445448c4c4f24f4f49f7cb1fdcd2f5362944606
diff --git a/services/tests/servicestests/src/com/android/server/notification/BuzzBeepBlinkTest.java b/services/tests/servicestests/src/com/android/server/notification/BuzzBeepBlinkTest.java
index 83a59fd..d51f2d8 100644
--- a/services/tests/servicestests/src/com/android/server/notification/BuzzBeepBlinkTest.java
+++ b/services/tests/servicestests/src/com/android/server/notification/BuzzBeepBlinkTest.java
@@ -76,6 +76,7 @@
         mService.setVibrator(mVibrator);
         mService.setSystemReady(true);
         mService.setHandler(mHandler);
+        mService.setSystemNotificationSound("beep!");
     }
 
     //