Android Question android uid system INCOMPATIBLE and system signature

[sqlcloner]

Hello to all . I have been involved in this issue for two weeks and I could not solve it. I will be happy to help you.
I use this code in my manifesto:

android: sharedUserId = "android.uid.system"

But when I use aosp signatures for my signature app. And I use Signapp.Jar to build my own Sign app. It gives the following error when running:

INSTALL_FAILED_SHARED_USER_INCOMPATIBLE

I had a few questions:
1: Where can I find Signapp.jar?
2: Where is this route? It is mentioned in various sites:
out / host / linux-x86 / framework / signapk.jar
3: I use the platform keys in the aosp to do this. Is this correct?

 

[KMatle]

You want to create a system app? I don't think that this is possbile as you would need the app to be signed by the manufacturer of the device/os like Samsung, etc. Otherwise anyone could replace any app.