Move SkPixelXorXfermode.h to include/client/android

This intended to make clients aware that the SkPixelXorXfermode is Android-specific

This needs to land after the Chromium CL https://codereview.chromium.org/1647953006/ (Add include/client/android to skia .gn & .gypi files)

GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1645093002

Committed: https://skia.googlesource.com/skia/+/aa9cb62901075e9d2858353cb041e5e6d4719614

Review URL: https://codereview.chromium.org/1645093002
diff --git a/public.bzl b/public.bzl
index 349d991..720a28e 100644
--- a/public.bzl
+++ b/public.bzl
@@ -335,6 +335,7 @@
 INCLUDES = [
     "include/android",
     "include/c",
+    "include/client/android",
     "include/codec",
     "include/config",
     "include/core",