Add Ethernet, TestNetworkSpecifier API

Rename StringNetworkSpecifier to EthernetNetworkSpecifier (its only
production user), and make it module-lib API.
The original StringNetworkSpecifier file is actually kept to satisfy
some invalid dependencies; it will be removed separately.

This allows specifying an Ethernet interface with a non-deprecated API:
until this change the only way to do so would be to use
NetworkRequest#setSpecifier(String), which is deprecated.

Similarly, add the TestNetworkSpecifier API for TestNetworkManager, to
replace previous usage of StringNetworkSpecifier. TestNetworkManager is
module API, so TestNetworkSpecifier should be module API too. This
allows tests to request the test interface specifically, without using
the deprecated NetworkRequest#setSpecifier(String).

Bug: 179329291
Test: m
Change-Id: Iee569f5c8bbdc4bc979610e1191308281f3d4620