This uses vk::Context to init DynamicDescriptorPool, and removes
some accesses to ProgramExecutableVk member variables so the methods
can more easily be moved into another class.

The changes should have no effective behaviour difference.

Note that the immutable sampler descriptor count is entirely set by
the format of the immutable samplers. However we can't determine this
count by looking at the descriptor set layout desc with the current
implementation.

Bug: angleproject:6776
Change-Id: I0a323d19fa3720b5afd8300401ed9c30b105c0ef
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3424657


Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Yuxin Hu <yuxinhu@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>