Add aconfig_declarations for flags in android_core_networking

Add aconfig_declarations so that platform code can use this flag.
Following CLs(aosp/2606670) use this flag to sync the mainline behavior
change and platform behavior change.
Having merged-in to avoid being merged to udc-mainline-prod where
aconfig soong targets are not supported

Bug: 267870186
Test: TH
Merged-In: I4870fbe882ce0ec995dcaeb8a2624071fee6dce7
Change-Id: I2260dc3d60689da9328fe4f98c3f3048d96bbf6d

common/Android.bp

@@ -19,6 +19,8 @@ package {
     default_applicable_licenses: ["Android-Apache-2.0"],
 }
 
+build = ["TrunkStable.bp"]
+
 // This is a placeholder comment to avoid merge conflicts
 // as the above target may not exist
 // depending on the branch

common/TrunkStable.bp

@@ -0,0 +1,21 @@
+//
+// Copyright (C) 2023 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+aconfig_declarations {
+    name: "com.android.net.flags-aconfig",
+    package: "com.android.net.flags",
+    srcs: ["flags.aconfig"],
+}