I have the same message, all things updated, turned phone in and off, still seeing the avastessage " malware detected in android system, it to s installed in the partition so cannot be uninstalled....setting to deactivate this app".
Well, I am not at all tekky, so spent an hour searching for this app, then searching for how to disable it, then googling how to disable it, then, I made the mistake of calling Google Support.....
Oh what a mistake, I spent over 2 hours, spike to 6 different people, including the manager of the support team responsible for my phone, and do you know what the result was....had I been able to uninstall it, I would have had a broken phone , because that is what they said to do, with the android system app.
I made it clear what it was, that it was a system file, sent them screenshots of the avast message, of tye app, of tye options available as they didn't know at all, and two told me to uninstall it and restart my phone, and 3 told me to disable or force stop.
I quickly realised they knew nothing, and turns out worse the advice given, if it had been possible to do so, would have made my phone unusable.
I then called my son who quickly said " don't touch it, leave it be, must be an error with avast, something in the latest Google update has something that avast is reading as a threat, even if it is a threat, don't touch the system files!"
Then I Google the error message from avast +mistake and found this.
So no, avast customer services, this issue is not resolved, all my definitions say up to date, all aps and is are up to date, yet avast is still telling me there is malware in the android system app and that I should disable it.
Luckily it doesn't tell you how to.
I am in the UK, to f that helps you get the definitions pushed out properly or whatever it is you do.