Added noop unregisterReceiver to complement existing noop registerReceiver.

Without this addition, code that is being tested using an isolated context
can cause an exception if it unregisters a previously registered broadcast
receiver.  This is because the isolated context never actually registered
the receiver in the first place.  The fix is to make sure the isolated
context is consistent in ignoring both recevier registration and
unregistration calls.

Change-Id: Ie0ba6f4bb10f5248704327a0ffc8e37ee8b71ae2

Author: jschwertfeger

test-runner/src/android/test/IsolatedContext.java

@@ -86,6 +86,11 @@ public Intent registerReceiver(BroadcastReceiver receiver, IntentFilter filter)
         return null;
     }
 
+    @Override
+    public void unregisterReceiver(BroadcastReceiver receiver) {
+        // Ignore
+    }
+
     @Override
     public void sendBroadcast(Intent intent) {
         mBroadcastIntents.add(intent);