Right now each shader stage has its own vk::DynamicBuffer for default
uniform storage. This is less efficient than just share one buffer. This
CL moves the storage from per shader stage into its ProgramVk object.

Bug: b/159457348
Change-Id: If47248ea23c4e48407d3b211583ae2b048d4d10f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2265281
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>