Update flags.aconfig to follow the current APIs

This CL also specifies aconfig_declaration module in java_sdk_library
register_nsd_offload_engine will be moved to the platform flags.aconfig
in the followup CL.

Test: TH
Change-Id: I430a5f0b1e1ef2e3394cf6aa8efebbd46da41765
diff --git a/Tethering/common/TetheringLib/Android.bp b/Tethering/common/TetheringLib/Android.bp
index 9c2a59d..47227e3 100644
--- a/Tethering/common/TetheringLib/Android.bp
+++ b/Tethering/common/TetheringLib/Android.bp
@@ -59,6 +59,9 @@
     lint: {
         strict_updatability_linting: true,
     },
+    aconfig_declarations: [
+        "com.android.net.flags-aconfig",
+    ],
 }
 
 java_library {