Previously ContextVk owned the descriptor pools. We were trying to
maximize descriptor reuse to conserve memory. However the default
uniforms would have no possible sharing. And because uniform buffers
are usually unique to a program it's likely there would be less reuse.

Image descriptors could be shared. But with the advent of a descriptor
cache in the Program it becomes difficult to manage the cache through
descriptor pool recycling. Moving the pools into the Program simplifies
the cache management. We could look at adding back more reuse in the
future.

Also shifts driver uniforms back into the end of the descriptor sets
to make indexing into the Program's descriptor pools simpler.

Bug: angleproject:3117
Change-Id: I52bb49cf322d944ad7cf08791efdf24b7fe573ce
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1644775
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>