dbus: transition to interface for testing

To make testing easier, the dbus objects now receive a
helper interface to use instead of calling into a helper module.

Tested: Ran on quanta-q71l board and it behaved as expected.

Change-Id: I2521d9c75aec76e92d2e26dc044d01011e44d552
Signed-off-by: Patrick Venture <venture@google.com>
diff --git a/dbus/dbusactiveread.cpp b/dbus/dbusactiveread.cpp
index 5e046a4..e41ba7a 100644
--- a/dbus/dbusactiveread.cpp
+++ b/dbus/dbusactiveread.cpp
@@ -27,7 +27,7 @@
     struct SensorProperties settings;
     double value;
 
-    GetProperties(_bus, _service, _path, &settings);
+    _helper->GetProperties(_bus, _service, _path, &settings);
 
     value = settings.value * pow(10, settings.scale);