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/framework-t/Sources.bp b/framework-t/Sources.bp
index 6652780..217a1f6 100644
--- a/framework-t/Sources.bp
+++ b/framework-t/Sources.bp
@@ -154,17 +154,17 @@
     ],
 }
 
+// TODO: remove this empty filegroup.
 filegroup {
     name: "framework-connectivity-tiramisu-sources",
-    srcs: [
-        ":framework-connectivity-ethernet-sources",
-    ],
+    srcs: [],
     visibility: ["//frameworks/base"],
 }
 
 filegroup {
     name: "framework-connectivity-tiramisu-updatable-sources",
     srcs: [
+        ":framework-connectivity-ethernet-sources",
         ":framework-connectivity-ipsec-sources",
         ":framework-connectivity-netstats-sources",
         ":framework-connectivity-nsd-sources",