wifi(API): NetworkSpecifier for Wifi NetworkAgent

Create an @hide NetworkSpecifier to use by the Wifi NetworkAgent. This
will be used by connectivity service to match the incoming
NetworkRequest (with WifiNetworkSpecifier) with the NetworkAgent we
created to serve that request.
The WifiNetworkAgentSpecifier will hold the current connected wifi
network configuration which will be used to pattern match the
WifiNetworkSpecifier from NetworkRequest's.

Also, added a @hide helper method in MacAddress to help with matching
bssid pattern.

Bug: 113878056
Test: Unit tests
Change-Id: I9a643f0b914d48ff64104c798ec2869db40cb24b