Add and plumb abstraction layer over GrDirectContext

Also changed GaussianBlurFilter's surface origin from
kBottomLeft_GrSurfaceOrigin to kTopLeft_GrSurfaceOrigin. This doesn't
seem to have an effect in practice, but aligns it with KawaseBlurFilter.

Additionally, both blur filters now set the protected bit on the
surfaces they create to reflect the protection status of the active
context, as opposed to either the protection status of the input SkImage
that is being blurred (Kawase) or always false (Gaussian). This should
be equivalent behavior in the case of Kawase (and aligns with Graphite),
and is likely a bug fix for Gaussian.

Test: manual validation (GL+VK) & existing tests (refactor)
Bug: b/293371537
Change-Id: I19b0258035ea5f319d04207ceb266f2cd1e87674