[graphite] Rename Metal classes to skgpu::graphite::Mtl<Class>

Change-Id: I76a01cf18583fab619459f258119b780ff978be5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/527840
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
38 files changed