Expose MacAddress methods to public API

To support mainline modules, adding the methods which
are used by wifi.

Bug: 139268426
Bug: 135998869
Bug: 138306002
Test: atest android.net.cts
      atest android.net.wifi.cts
      atest FrameworksNetTests
      atest NetworkStackTests
      ./frameworks/opt/net/wifi/tests/wifitests/runtests.sh

Change-Id: I49856863c15b6b3b59aa867a77b0f137003149ef