shill: Broadcast service strength changes.

Similar to flimflam, make strength a property of the base service class, and
broadcast changes to the property.

BUG=chromium-os:25011
TEST=unit tests, tested on device

Change-Id: Iff7a638a2c1b8a828260739273c428373aff420e
Reviewed-on: https://gerrit.chromium.org/gerrit/14054
Commit-Ready: Darin Petkov <petkov@chromium.org>
Reviewed-by: Darin Petkov <petkov@chromium.org>
Tested-by: Darin Petkov <petkov@chromium.org>
diff --git a/manager_unittest.cc b/manager_unittest.cc
index d4f54b1..78d332b 100644
--- a/manager_unittest.cc
+++ b/manager_unittest.cc
@@ -764,7 +764,7 @@
   EXPECT_TRUE(ServiceOrderIs(mock_service0, mock_service1));
 
   // Two otherwise equal services should be reordered by strength
-  mock_service1->set_strength(1);
+  mock_service1->SetStrength(1);
   manager()->UpdateService(mock_service1);
   EXPECT_TRUE(ServiceOrderIs(mock_service1, mock_service0));