Add SkiaVk backend to RenderEngine

This CL adds a new backend, SkiaVk, to RenderEngine.

The new functionality is to create a Vulkan device/instance (possibly
protected) and handle flush/waitFence via VkSemaphores fed to
GrBackendSemaphores.

+ make ctors of GLES/Vk RE's private so as to ensure GrContexts are
  created

Test: atest librenderengine_test
Bug: 236390072

Change-Id: I69119623b194885bcc4cf2ddc8e592576b713b19