Enable multiple active Ethernet interfaces

- add Ethernet interface configurations to config.xml; no vendors can
specify network capabilities (in particular they can mark network as
restricted which make sense for embedded applications + static IP
configuration)
- extend EthernetManager to support multiple interfaces, use interface
name as an identificator
- extend IpConfigStore to store IP configuration based on string
identifier (e.g. ethernet name)

Test: runtest -x frameworks/base/services/tests/servicestests/ -c com.android.server.net.IpConfigStoreTest
Change-Id: Ic1e70003f2380ca8edb4469d6b34e27c5e8cf059