Cleanup mip-mapped proxy creation slightly

Cherry-pick of https://skia-review.googlesource.com/c/skia/+/154305
to android/next-release. First of two changes to support fallback
for uploading F16 bitmaps.

No-Tree-Checks: true
No-Try: true
No-Presubmit: true
Bug: skia:8375
Change-Id: Ie0b1468839418032fa882f08c5d9a16de97ac38a
Reviewed-On: https://skia-review.googlesource.com/154305
Reviewed-By: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-on: https://skia-review.googlesource.com/154621
Reviewed-by: Brian Salomon <bsalomon@google.com>