More refactoring for Viewer

* Move support files into sk_app and main files up to top directory
* Rename VulkanTestContext and create WindowContext parent class
* Place VulkanWindowContext et al. in sk_app namespace.
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1944413005

Review-Url: https://codereview.chromium.org/1944413005
23 files changed