Update extra info before registering

The extra info is taken into NetworkMonitor from while creating
it. The NetworkMonitor is created when a new agent is registered
but the extra info is not available at that time. Make sure the
field is set in the NetworkInfo when registering.

Bug: 156173829
Test: adb shell dumpsys network and check the apn in the extra
      info shown correctly
Test: atest FrameworkNetTests
Change-Id: Ieaad8cbf1a28af3b97c7f98f74358e417fcad661