Alternative fix for stale MIP maps on texture export/import.

This change assumes imported textures in SkImages have dirty mip levels
and disables regenerating the MIP levels on export (added in
https://skia.googlesource.com/skia/+/930f939c48af1be6005af12bb6f709ca5da9d118).

This is being done temporarily because it is a simpler change to cherry
pick to earlier branches than
https://skia.googlesource.com/skia/+/930f939c48af1be6005af12bb6f709ca5da9d118

No-Tree-Checks: true
No-Try: true
No-Presubmit: true
Bug: chromium:850617
Change-Id: I7c90da782a0ed9014acd6a54ffdee1a0b34fa00c
Reviewed-On: https://skia-review.googlesource.com/136788
Commit-Queue: Brian Salomon <bsalomon@google.com>
Auto-Submit: Brian Salomon <bsalomon@google.com>
Reviewed-By: Greg Daniel <egdaniel@google.com>
Reviewed-on: https://skia-review.googlesource.com/137131
Reviewed-by: Brian Salomon <bsalomon@google.com>