Fix update_scheduler_enums.py

It had a bad regex for the finding the start of the enum.

Expose the various constants from update_scheduler_enums.py so that they
can be reused in the PRESUBMIT where they were *also* incorrect.

I manually tested that the PRESUBMIT now catches changes to the source
enum that are missing updates to the XML.

Also change update_histogram_enum.py to ignore min/max entries,
otherwise it complains about duplicates.

Add/fix enum entries in enums.xml .

Add `LINT.IfChange` protection for this enum.

NO_IFTTT=IFTTT is being introduced in this CL.

Change-Id: Ieac44d1b30e75f38d8a6f186248be61871ec9d7f
Bug: 419089558,41495725
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6656361
Reviewed-by: Luc Nguyen <lucnguyen@google.com>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Auto-Submit: Fergal Daly <fergal@chromium.org>
Reviewed-by: Mingyu Lei <leimy@chromium.org>
Commit-Queue: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1477884}