Split out the ethernet API surface and use framework-connectivity-t

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: I3e13cb9c0e348333af295c2537d459aa6700ff17
Merged-In: I1956848d3248cc56e9841d221e5e4c160bed65a4
diff --git a/service-t/Sources.bp b/service-t/Sources.bp
index 5100e7c..4b799c5 100644
--- a/service-t/Sources.bp
+++ b/service-t/Sources.bp
@@ -102,17 +102,16 @@
     ],
     path: "src",
     visibility: [
-        "//frameworks/opt/net/ethernet",
+        "//frameworks/opt/net/ethernet/tests",
     ],
 }
 
 // Connectivity-T common libraries.
 
+// TODO: remove this empty filegroup.
 filegroup {
     name: "services.connectivity-tiramisu-sources",
-    srcs: [
-        ":services.connectivity-ethernet-sources",
-    ],
+    srcs: [],
     path: "src",
     visibility: ["//frameworks/base/services/core"],
 }
@@ -120,6 +119,7 @@
 filegroup {
     name: "services.connectivity-tiramisu-updatable-sources",
     srcs: [
+        ":services.connectivity-ethernet-sources",
         ":services.connectivity-ipsec-sources",
         ":services.connectivity-netstats-sources",
         ":services.connectivity-nsd-sources",