add SkStubHeifDecoderAPI.h

This should let us build SkHeifCodec even without libheif.

I didn't look at libheif itself, so this stub may disagree
a bit with the actual interface... just flailed around until
it compiled and linked.

This turns on the stub in most of our internal builds.

Change-Id: I62ed4993198118b2a8217ec846d92ff637cc8ab9
Reviewed-on: https://skia-review.googlesource.com/62321
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
diff --git a/BUILD.gn b/BUILD.gn
index 2b333af..03a11a6 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -31,7 +31,7 @@
   skia_use_piex = !is_win
   skia_use_zlib = true
   skia_use_metal = false
-  skia_use_libheif = false
+  skia_use_libheif = is_skia_dev_build
 
   skia_android_serial = ""
   skia_enable_discrete_gpu = true