Make an Android-specific copy of Skia's Null GL interface

This functionality has been removed from Skia, but is still being used
by HWUI's NullGlesDriver for unit tests. Making a local copy gets tests
passing (so that rolling can resume).

Test: hwui_unit_tests still pass
Change-Id: I16b65af58eaeca5a39df89d1542760b652d8b0d5
diff --git a/libs/hwui/Android.bp b/libs/hwui/Android.bp
index ebba4cb..9fe53d3 100644
--- a/libs/hwui/Android.bp
+++ b/libs/hwui/Android.bp
@@ -101,6 +101,7 @@
         "debug/GlesDriver.cpp",
         "debug/FatalBaseDriver.cpp",
         "debug/NullGlesDriver.cpp",
+        "debug/NullSkiaInterface.cpp",
     ],
     include_dirs: ["frameworks/native/opengl/libs/GLES2"],
 }