Internal API for system apps to determine default network for other apps

Callers with CONNECTIVITY_INTERNAL permission can read off the netId
that an application is assigned to by default.

Necessary for making connections as the default network for a client
app eg. when downloading stuff on its behalf.

Bug: 27074270
Change-Id: I8d35e8e99126875f55f3c545090326f3e9be43fb