Propagate protectedness to SkPictureShader draws

This change seems harmless and is more correct. Basically, the protected-ness of the backing surface is propagated to the internal image created for a PictureShader. This is relevant if the SkPicture contains protected content.

Bug: b/513969378
Change-Id: I9314aca6481850d098bab0515601cca51fb7a00a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1281217
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>