Move SkiaGpuContext creation in SkiaVkRenderEngine to virtual function

Making GaneshVkRenderEngine / GraphiteVkRenderEngine handle this reduces
the need for higher-level branching.

Test: boots successfully with skiavkthreaded
Bug: b/293371537
Change-Id: I5685cb9dd7b0dc1ce3a2d538a729c255fbd6c52b
4 files changed