Add tests to add/remove/re-add a NAT.

This is in preparation of work to support multiple upstreams.

Test: netd_unit_test still passes with current behavior.
Bug: 38218697
Bug: 64382985
Bug: 64976379
Bug: 64995262
Change-Id: Idc7f9a1eeea435f4eade8fec9471bbc3c78ed01d
Merged-In: I202b19c3ff8d69b6b4bb73679618f4685b043f77
Merged-In: Ida97fd8379720b7dc083e2407c667c7d61181365
(Cherry-picked from aosp 70a269a7)