Add support for enabling GraphiteVkRenderEngine in SurfaceFlinger

Predicated on just the graphite_renderengine flag (overridable with
the debug.renderengine.graphite sysprops), which inherently implies
GraphicsApi::VK.

As currently implemented, setting the old debug.renderengine.backend
sysprop (e.g. "skiavkthreaded") will fully override this logic,
preventing Graphite from being enabled. This approach is simpler, but
also maintains backwards compatibility, and distances the new approach
from that restrictive terminology.

Note that the approach for actually instantiating a RenderEngine
instance extends the existing RenderEngine::create(args) model, forcing
the caller to know exactly which concrete subclass they wish to
instantiate. This could be reworked in the future.

Test: local validation (sysprops + logs)
Bug: b/293371537
Change-Id: Iff0a8894725fc45e679074ddc2e4af56cca318c6