Migrate Serial from Image to ImageView.
Imageviews are what are utimately used in FramebufferVk, so move
the Serials into the ImageViewHelper class. Since that class also
knows the level/layer of the imageView, we can revert to using a
single Serial per ImageView instead of the AttachmentSerial that
included the layer and level.
ImageViewHelper caches Serials per layer/level combo.

Bug: angleproject:4651
Change-Id: I3741d7d03523eada84295cb712c1cc1e6e3c3867
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2248203
Commit-Queue: Tobin Ehlis <tobine@google.com>
Reviewed-by: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>