framework: Introduce INetworkManagementService for communicating with netd

This is the framework-exposed interface for the (future) management of
all Android network management functions. Initial support is for providing
tethering.

Updated: Integrate feedback from review; clean up naming

Updated: Switch from add/remove/list to get/set for DNS forwarders
         - allows prioritization / ordering of DNS servers

Updated: Refactor NAT api

Updated: Refactor NAT api (last time sorry)

Signed-off-by: San Mehat <san@google.com>