Add kRenderTarget_GrSurfaceFlag to GrVkGpu::onWrapBackendRenderTarget

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4868

Change-Id: I7f169d050c47c074cf63ef9b63fb7df8efdaed62
Reviewed-on: https://skia-review.googlesource.com/4868
Commit-Queue: Robert Phillips <robertphillips@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>