Revise Text & Small Path Atlas so instantiation failure is handled at flush time

This paves the way to having the AtlasTextOps not need the RestrictedAtlasManager at op creation time.

Change-Id: I1028faba730d50d3d3349a4c0809465d036ed611
Reviewed-on: https://skia-review.googlesource.com/111807
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>