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/ISurfaceComposerClient.cpp b/libs/gui/ISurfaceComposerClient.cpp
index f199e9f..de4d8c1 100644
--- a/libs/gui/ISurfaceComposerClient.cpp
+++ b/libs/gui/ISurfaceComposerClient.cpp
@@ -114,7 +114,7 @@
             status_t result = createSurface(name, w, h, format, flags,
                     &handle, &gbp);
             reply->writeStrongBinder(handle);
-            reply->writeStrongBinder(gbp != NULL ? gbp->asBinder() : NULL);
+            reply->writeStrongBinder(IInterface::asBinder(gbp));
             reply->writeInt32(result);
             return NO_ERROR;
         } break;