Change list + manual search to set/map

Continued incremental cleanup to simplify change to iptables restore.
Rename some data members and switch to better data structures.

Test: as follows
    - built
    - flashed
    - booted
    - "runtest -x .../netd_unit_test.cpp" passes
    - "runtest -x .../netd_integration_test.cpp" passes
Bug: 28362720
Bug: 38143143

Change-Id: Iff231bf180f9195b01e09c5cb8c883c5d3f2852a