Convert the Bluetooth NetworkAgent to the @SystemApi way.

This is a functional no-op but abandons using the old hidden
API for NetworkAgent.

Tag: #refactor
Test: manual – this device can still connect to another device
      sharing its connection via Bluetooth and access the internet.
Bug: 150570873
Change-Id: Ia52343debcba4b873f817f9b3e97a0b84cc985f1