Remove hidden API usage in metrics toString

Remove usage of hidden NetworkType, transport APIs in toString()
implementations of metrics and data usage classes.

The toString implementations can log the transports or network type as
hex or raw indices. While slightly less readable, the metrics classes
and network type APIs are deprecated.

Bug: 174436414
Test: m
Change-Id: I79239a540b66dadd3bbe0a997960530878331358