Clean ExternalMemoryUtil

Clean ExternalMemoryUtil

This change separates choosing of memory type from
allocateExportableMemory utility function. This makes
use of this functions more clear.
Previous implementation needed to be done ASAP and
involved passing of non-const reference to
allocateExportableMemory which was then filled.
This workaround made code unnecessarily hard to follow.

Components: Vulkan

VK-GL-CTS issue: 587

Affects:
dEQP-VK.api.external.memory.*
dEQP-VK.synchronization.signal_order.*
dEQP-VK.synchronization.cross_instance.*

Change-Id: I781388db390f13c7c03785dd0ae34ae0128ec31c