Ethernet framework and service source code is going to be moved to
Connectivity mainline module, this CL contains below corresponding
changes to adapt the migration in f/b side:

1. Split out ethernet module-lib APIs to Connectivity module.
   Add the ethernet resource filegroup to tiramisu-updatable-sources
   filegroup as well, build them together. Also update the module-lib
   and system api txt to reflect the APIs change. Remove the hidden
   APIs which are moved to Connectivity module. This removal fixes the
   api inconsistent issue with Tethering/apex/hidden/.

2. Remove EthernetService from SystemServer which will be registered
   from ConnectivityServiceInitializer.

3. Replace the BackgroundThread class(imported from f/b/core/java)
   with the one in the modules-utils-backgroundthread lib, which is
   visible to Connectivity module.

Bug: 210586283
Test: m
Test: atest FrameworksNetTests EthernetServiceTests
Change-Id: I1956848d3248cc56e9841d221e5e4c160bed65a4