Export TARGET_USES_VULKAN as string

Make build.prop has three values for ro.hwui.use_vulkan
- true
- false
- (empty string)

In preparation for validating soong built vendor filesystem, export
TARGET_USES_VULKAN as a string. This allows `gen_build_prop.py` to
create empty `ro.hwui.use_vulkan` (currently it will populate it as
false)

Bug: 382295862
Test: compared vendor build.prop for aosp_cf_x86_64_phone
Change-Id: Ib6a37a950a33e7b06c260bb651347b4fb4e36834