Allow overlap in resolv uid => DNS iface mapping

When multiple rules exist covering a given uid the one added most
recently will be used.

This allows us to handle the simultaneous tuns case where a new tun is
coming online for an already running VPN.

_resolv_clear_iface_for_uid_range now also takes the iface and removes
only that matching (iface, uid range) entry.

Bug: 12134439
Change-Id: I9b9cfcfae2f38c409022a8c76ccadad7e2babd78