OS battery charge limit causes scheduled backups (with charging constraint) to fail

c.f. https://github.com/GrapheneOS/os-issue-tracker/issues/4626

the issue (Charge limit causes bug with Seedvault device backup)

How to reproduce

1. Enable the charge limit
2. Plug the phone to charger
3. (Maybe unnecessary) Go to "Backup scheduling" > enable "Back up only when charging"
4. Enable "Backup scheduling"
5. When the scheduled backup happens, the notification "backup failed" happens

Effect

This causes the "backup failed" notification to pop up, and come back whenever it gets dismissed.

Temporary solution

Turn off charge limit, or unplug the phone, or turn off backup scheduling and run backups manually instead.

Originally posted by @schklom in https://github.com/GrapheneOS/os-issue-tracker/issues/4626#issue-2767895555

and the official reply

Needs to be fixed in Seedvault upstream. We're using the standard Android HAL for this.

Originally posted by @thestinger in https://github.com/GrapheneOS/os-issue-tracker/issues/4626#issuecomment-2569552381

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EDIT follow-up messages on original issue:

We're planning on replacing Seedvault with an app using a significantly different approach and don't really want to track the issues here.

Originally posted by @thestinger in https://github.com/GrapheneOS/os-issue-tracker/issues/4626#issuecomment-2569568579

There's a standard Pixel HAL feature for setting the charge to limit just to clarify what is meant by that. We're not setting it with the low-level kernel APIs which are not properly supported by the higher level OS. We considered doing it that way to make it more configurable but it's not the right approach without significant further changes.

Originally posted by @thestinger in https://github.com/GrapheneOS/os-issue-tracker/issues/4626#issuecomment-2569595452