Use InterfaceSet for upstream interfaces.

Allows using different upstream interfaces for IPv4 and IPv6.

Bug: 38218697
Bug: 64382985
Bug: 64976379
Bug: 64995262
Bug: 64380515
Test: runtest frameworks-net, manual (with aosp/644099 applied)

Change-Id: I3db63f7aa5255a0229253187def7590a386c5133