Start service from system app in a custom rom

Question

My usecase is really simple, I want to create a system app which starts a Service. (I'm new to custom-rom development)

To do so I've added: android:sharedUserId="android.uid.system" in my AndroidManifest.xml. As of now I'm still using an official Android version in my emulator (which is probably why I'm getting this issue)

I start my service with:

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O) context.startForegroundService(service);
else context.startService(service);

Unfortunately the service doesn't start and I get the following error:

Calling a method in the system process without a qualified user: 
android.app.ContextImpl.startForegroundService:1466 
android.content.ContextWrapper.startForegroundService:649

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EDIT: When looking to the source file the warn comes from the startForegroundService which does:

@Override
public ComponentName startService(Intent service) {
    warnIfCallingFromSystemProcess();
    return startServiceAsUser(service, mUser);
}

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EDIT2: After some more investigation, I think the issue comes from

type=1400 audit(0.0:31): avc: denied { create } for name="mgmtsocket" scontext=u:r:system_app:s0 tcontext=u:object_r:system_app_data_file:s0 tclass=sock_file permissive=0

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EDIT3: More informations:

• I'm testing against a rooted emulator which runs on Android 8 API 26 (official version)
• I've signed my app using the AOSP key from this and using the following command line apksigner sign --key platform.pk8 --cert platform.x509.pem filename.apk
• Concerning my permissions and service I have:

<uses-permission android:name="android.permission.INTERNET" />
<uses-permission android:name="android.permission.FOREGROUND_SERVICE" />

<service
    android:name=".MyService"
    android:permission="android.permission.BIND_VPN_SERVICE"
    android:exported="false">
    <intent-filter>
        <action android:name="android.net.VpnService" />
    </intent-filter>
</service>