Move several config elements into TetheringConfiguration class

Encapsulate the following config elements:
    - all interface name regular expressions
    - specification of preferred upstreams
    - downstream DHCPv4 ranges
    - default IPv4 (and IPv6) DNS servers

Test: as follows
    - builds
    - flashed (bullhead)
    - runtest frameworks-net passes
Bug: 32163131
Change-Id: Ife4871c0b89d2d52e2cc9238e0606b428287521c