Migrate from GrContext to GrDirectContext

This is part of an effort to distinguish between different types of
contexts in GPU Skia. When using a DeferredDisplayList (DDL)
recorder, the context you get is not a direct context and cannot
be used for operations like uploading or reading textures.

Since Android does not use DDLs, it is not directly affected by this
change but other APIs, such as SkImage::MakeFromTexture are
being migrated to require a GrDirectContext to increase sanity.

Change-Id: I9afbdf3c026a9f9cb6ad2aad904915e189e584d6