The presence of a special Gradle property is now used to exclude Vulkan
support from the build. By making Vulkan "always on" for local
development, we can avoid stale CMake cache issues that arise from
toggling the Gradle property. It also lets you avoid adding the flag
to Android Studio preferences.
This is motivated by testing and does not indicate production readiness.
Clients still need to pass VULKAN into the Engine constructor to select
the Vulkan backend.
To keep APK size down and keep CI fast, we are continuing to exclude
Vulkan from official Android builds.
After syncing this change, you might need to use `./build.sh -c` to
clobber various build caches.
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