Register ethernet listeners on specific Executors

Have users of EthernetManager#addListener specify an Executor instead of
running callbacks on ConnectivityThread.
For existing clients, run callbacks on the common BackgroundThread.

This change in behavior is fine because addListener is not an API, the
threading model in Q was to run callbacks on the looper of the thread
that first created EthernetManager (so there is already no guarantee on
which thread is used), and the only users are in Settings.

ConnectivityThread is moving to a connectivity-specific JAR, so it
should not be used by external classes.

Bug: 174436414
Test: Ethernet client / tethering working
      atest TetheringIntegrationTests
Change-Id: Ic9a959f89915329f79eeeaa0a210ad50811d1291