Tests just need a specifier to verify general NetworkSpecifier
functionality. They can use EthernetNetworkSpecifier the same way they
use StringNetworkSpecifier.
To keep the tests passing on Q and R, add a CompatUtil class that will
keep creating StringNetworkSpecifiers (now removed) instead.
Bug: 179329291
Test: atest CtsNetTestCases
Merged-In: Ic6bf611ba36b555e4e0bb1bd3fa0053fe29dc8cf
Change-Id: I38fa9eed5915264a53cc70e8f170c003eefe04e2
71de00d339