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/IGraphicBufferProducer.cpp b/libs/gui/IGraphicBufferProducer.cpp
index 1e28f9b..bcdf368 100644
--- a/libs/gui/IGraphicBufferProducer.cpp
+++ b/libs/gui/IGraphicBufferProducer.cpp
@@ -211,7 +211,7 @@
         data.writeInterfaceToken(IGraphicBufferProducer::getInterfaceDescriptor());
         if (listener != NULL) {
             data.writeInt32(1);
-            data.writeStrongBinder(listener->asBinder());
+            data.writeStrongBinder(IInterface::asBinder(listener));
         } else {
             data.writeInt32(0);
         }