Use onNetworkUnwanted() instead of unwanted()

NetworkAgent#unwanted() is a hidden API which cannot be called
after putting NetworkAgent into mainline module, so the caller
should use the public one - NetworkAgent#onNetworkUnwanted()
instead.

Bug: 178991503
Test: Add a log and connect to a network which is provided by
      another device through BT tethering. Disconnect the BT
      network and check the log.
Change-Id: Ib341289366e2c1c2ac8e653ccddd9df81b2084d4