Disable SkiaVK support on CPU implementations

Swiftshader is not ready for SkiaVK and causing black
screen problems when enabled. We temporarily disable
the support for CPU device types as a workaround until
the actual problems are fixed.

Bug: 326633110
Test: emulator -gpu swiftshader
Change-Id: I76c4347c3f9ffd9ddb4f85dae0e05eba191c0069