For vkImages that are backed by external memory we need to derive
the VkImageUsageFlags based on the usage flags of the AHB.

Bug: angleproject:4739
Change-Id: I92bee943c66f7eadd9b63bb9bf1b59ffe5bc5a37
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2243765Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>