Split Ganesh-specific parts of SkiaVkRenderEngine into subclass

Similarly to Ia65306cc825b71fe0b89c7f8545ce1c71a81d86b, this will allow
for a Graphite-specific variant of SkiaVkRenderEngine.

waitFence and flushAndSubmit are kept on the *RenderEngine classes (as
opposed to the recently added SkiaGpuContext abstraction) because they
are specific to the intersection of both <Skia backend> x <GPU API>,
with 3 variants. Their logic is also RE-specific, and not suitable for
a Skia abstraction.

Misc. cleanup:
- Remove SkiaVkRenderEngine's flush() and mBackendContext declarations
  as they were undefined and unnecessary.
- Mark some local variables as const.
- Pass named constant to GrDirectContext::wait in
  GaneshVkRenderEngine::waitFence

Test: manual validation & existing tests (refactor)
Bug: b/293371537
Change-Id: I695a0554b20ca14cf475aeff1b92c4445c1af55b