Replace IInterface::asBinder() with a static

so we can do NULL checks again, and update calls to IInterface::asBinder()
to use the new static version.

Change-Id: Ia7b10eb38ca55b72278bfd33d3bf647f338b4e6a
diff --git a/libs/gui/SensorManager.cpp b/libs/gui/SensorManager.cpp
index 7b4fa2f..17960ff 100644
--- a/libs/gui/SensorManager.cpp
+++ b/libs/gui/SensorManager.cpp
@@ -86,7 +86,7 @@
         };
 
         mDeathObserver = new DeathObserver(*const_cast<SensorManager *>(this));
-        mSensorServer->asBinder()->linkToDeath(mDeathObserver);
+        IInterface::asBinder(mSensorServer)->linkToDeath(mDeathObserver);
 
         mSensors = mSensorServer->getSensorList();
         size_t count = mSensors.size();