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
7 files changed