Add support to set a default device type
using existing ANGLE_DEFAULT_PLATFORM
environment variable. The valid vulkan options
are as follows:

vulkan              - vulkan with device hardware
vulkan-null         - vulkan with null device (Mock ICD)
swiftshader         - vulkan with swiftshader

Bug: angleproject:3998
Change-Id: I15f00e8024818fbaf674ca6fcdbcdac5a90cace2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1956140
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>