WifiConfiguration: add local only networkId

When applications start LOHS, they need a way to connect to the
network on the same device.  The WifiConfiguration.networkId
will be set to the newly defined value to serve as a signal to
ConnectivityService that this request is for a local only
network connection.

Bug: 62076211
Test: frameworks/base/wifi/tests/runtests.sh
Change-Id: Iab5240a7b4f526f8bc72dd44cdba59e65faedcd1