Move vulkan external commandbuffer logic from RT to GrVkFramebufer.

Bug: skia:11809
Change-Id: I7fa364f7472be35e02b06fa2f3fbb6d61a427f65
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/396156
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
8 files changed