[graphite] Update naming to reflect that VulkanFramebuffers can be cached

* Since https://skia-review.googlesource.com/c/skia/+/949656, we do cache framebuffers. Update method names + comments to reflect this.

Change-Id: Id768ed094456308baeeb872c33b1d9bae8c96666
Bug: b/302126809
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/994557
Auto-Submit: Nicolette Prevost <nicolettep@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Nicolette Prevost <nicolettep@google.com>
Commit-Queue: Michael Ludwig <michaelludwig@google.com>
3 files changed