Cache VkImageCreateFlags in ImageHelper. This allows texture target
siblings to inherit a few VkImage create attributes thus avoiding
image respecification in certain code paths.

Bug: angleproject:2514
Bug: angleproject:5281
Test: PbufferTest.ClearAndBindTexImageSrgbSkipDecode*Vulkan
Change-Id: Ic7397fabdce185264e06488355ca47f809338519
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2910473
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>