Allow underlying cell without unwanted NOT_VCN_MANAGED capability

This change updates the UnderlyingNetworkTracker to rely on the
TelephonyNetworkSpecifier for selecting underlying networks. This
ensures that there is always a cellular NetworkRequest filed for
Cellular to bring up an underlying Network.

Bug: 182211364
Test: FrameworksVcnTests
Change-Id: I826711bb671440054c2090793315a5f1d57596c0