Set min_sdk_version to be part of mainline modules

Modules contributing mainline modules (APK/APEX) should set
min_sdk_version as well as apex_available.

For now setting min_sdk_version doesn't change build outputs.
But build-time checks will be added soon.

Exempt-From-Owner-Approval: cp from aosp

Bug: 152655956
Test: m
Merged-In: Idc4aa437c21ea1add95a709bf709a5dbe662b084
Change-Id: Idc4aa437c21ea1add95a709bf709a5dbe662b084
(cherry picked from commit d3fb9f5bb31766291e7f7e506cfb1a17b4c70d1c)
diff --git a/Android.bp b/Android.bp
index 008c56d..81a96a8 100644
--- a/Android.bp
+++ b/Android.bp
@@ -220,7 +220,8 @@
     srcs: [
         "jni/network_stack_utils_jni.cpp"
     ],
-    sdk_version: "current",
+    sdk_version: "29",
+    min_sdk_version: "29",
     shared_libs: [
         "liblog",
         "libnativehelper_compat_libc++",