Register NetworkCallbacks in UnderlyingNetworkTracker.

This CL updates UnderlyingNetworkTracker to register NetworkCallbacks.
UnderlyingNetworkTracker uses two types of NetworkCallbacks:
  - bringup requests
  - selection requests

Bringup requests ensure that VCN-managed Networks are not reaped.
Separate bringup requests are filed for wifi and cell Networks. One cell
request is filed for each subscription ID (subId) in the VCN's
subscription group.

Selection requests are used to allow ConnectivityService to perform
Network selection for the VCN's selected underlying Network (the Network
that VCN traffic will be routed through).

Bug: 177364490
Test: atest FrameworksVcnTests
Change-Id: Ic4dc13a3f7acf08589ddb25b343af2677cd7cf65