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)
(cherry-pick of aosp I3db63f7aa5255a0229253187def7590a386c5133)
Merged-In: Ic8602dd029b5a9626aaf6651315b82ab6327037c
Merged-In: I015ac946afa581be1d94f10ab766ac00f5a4ede0

Change-Id: I8c48dab5eb67e60dadbfa974122fc884f4badd15