Make some of the UID-based VPN code reusable

By changing some member refs into arguments and having one of the
functions create the UID range instead of adding to mVpnUsers.

This will be useful for other layers of UID filtering like having
UIDs explicitly blocked from the VPN.

Deleted one broken line of code that cleared the status intent when
a restricted profile is removed. Other than that, this commit shouldn't
change any behaviour. If it does, that's a bug.

Bug: 26694104
Change-Id: Ieb656835d3282a8ba63cc3f12a80bfae166bcf44