Set extraInfo on cellular TestNetworkAgents.

This makes the test more realistic, since telephony always puts
the APN name in the extraInfo field. It also makes it easy to
test that various APIs properly return the extra info without
plumbing a new parameter through TestNetworkAgentWrapper,
NetworkAgentWrapper, and InstrumentedNetworkAgent.

Also make the extraInfo and the legacyType available to tests.
This will be used in future tests that assert the contents of
extraInfo.

Bug: 181855958
Test: test-only change
Change-Id: If4eddb6cd9e70bb33e10d72aceebaea843244246