Merge duplicate APIs in VulkanInterface

Now that the Ganesh and Graphite share the same VulkanBackendContext
type, the two `getGaneshBackendContext` and `getGraphiteBackendContext`
functions on RenderEngine's VulanInterface can be merged.

I've tweaked the name to `createSkiaVulkanBackendContext` for clarity.

Bug: b/309785258
Test: compiles (refactor)
Flag: EXEMPT trivial refactor
Change-Id: I84aa3f4f2e06cf660dfc13be68795843596d0809