The code for initializing a VkImage with VK_KHR_image_format_list
extension support is now internal to initExternal. Also provides a flag
to inform callers when VK_KHR_image_format_list support is enabled.

Bug: angleproject:5520
Change-Id: Ie2ade93d1403ab3f9cbcf7d80684bbca201d5d8d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2702159Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>