Rename VulkanViewer to Viewer, take 2.

Also puts Window and Application in sk_app namespace.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1945103003

Review-Url: https://codereview.chromium.org/1945103003
31 files changed