Add new GrVkBackendContext::Create explicitly requiring vk proc getters

Add new GrVkBackendContext::Create explicitly requiring vk proc getters

Also remove the feature of GrVkGpu that creates the instance/device if the client doesn't provide one.

Change-Id: Ie617313b6c684ed355333a475b80d0aae7e3a026
Reviewed-on: https://skia-review.googlesource.com/14261
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>