Don't use @hide API - ConnectivityManager#getNetworkTypeName()

To prevent calling @hide API, print definition of network type
directly instead of calling @hide API.

Bug: 172183305
Test: atest FrameworksNetTests
Test: atest CtsNetTestCasesLatestSdk
Test: Setup a platform VPN and enable VPN always-on, then check
      the log.
Change-Id: I807e90537f706face51c5cc221490ea742d22a0e