Right now staging buffers are per BufferVk. This will make it per
ContextVk so that it can be shared among all objects that needs a
staging buffer.

Bug: b/161846868
Change-Id: I9c436acdacaf429a43cbdfa216927df0796f7a28
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2310962Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Ian Elliott <ianelliott@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Charlie Lao <cclao@google.com>