Only send to downstream ifaces for null network

When sendMulticastPacket or sendUnicastPacket is called with the null
network, only send the packets to interfaces indexed with null, instead
of all interfaces.

When using MdnsMultinetworkSocketClient sending packets on the null
network means sending on tethered downstream interfaces. When
MdnsSocketClient is used (not used in the Android tree), sending on the
null network sends on all interfaces as MdnsSocketClient does not
support specific networks. This is clarified by explicitly throwing
when a non-null Network is attempted to be used with MdnsSocketClient
(but MdnsSocketClient is only used for tests in the Android tree).

Bug: 283708537
Test: atest
Change-Id: Ia0186bf8aa2e0fc5878d6071fd23599df8488616