Use unmodified TARGET_USES_VULKAN in ro.hwui.use_vulkan

This will create an empty ro.hwui.use_vulkan if TARGET_USES_VULKAN is
empty.

This matches the make behavior, and helps with validating the contents
of soong vendor filesystem.

Bug: 382295862
Test: lunch aosp_cf_x86_64_phone-trunk_staging-userdebug
Test: m out/soong/.intermediates/build/soong/fsgen/aosp_cf_x86_64_phone_generated_vendor_image/android_common/aosp_cf_x86_64_phone_generated_vendor_image.img
Test: Verified that ro.hwui.use_vulkan is same between make and soong
(empty string)

Change-Id: I9f80e613f7539dd2e895cfa96bd2ba481ddd6974