Plugin/theme auto update already on by earlier filter

[jeffpaul]

Migrated from: audrasjb/wp-autoupdates#16
Previously opened by: @knutsp
Original description:

When add_filter( 'auto_update_{plugin|theme}', '__return_true', 1 ); so that $update is already true, wp-autoupdates should leave it as is for backwards compatibility with plugins. This should be detected so that there is no "Enable" action button, and probably no "Disable".

It could be nice to be able to override this by allowing "Disable", though.

I use "Advanced Automatic Updates" by pento and I expect it to still work without intervention and indicated correctly in the plugins list.

[jeffpaul]

Previously commented by: @knutsp
Original comment:

"Advanced Automatic Updates" with plugins checked has no effect on the list now. I still think this should show all plugins as enabled, but with no visible disable action, or subsidiarly disable action just to have no effect.

[getsource]

I'd also like to see the previous filters work as currently expected by users.

A new-for-this-feature filter (or other method) that allows turning on auto-updates for new plugins by default (if this doesn't already exist) would be helpful as well.

[pbiron]

For context, see audrasjb/wp-autoupdates#10 (comment) (my comment on a PR in the old repo for this feature plugin...that didn't get migrated over to this repo).