Set module min_sdk_version to 28

The modules are not actually compatible with 28, but this is necessary
as tooling does not support "Q" as a version in the APK.

This will need to be changed to the new version number as soon as it is
available.

Test: m
Change-Id: I1d88f80c293b4667244d219a6989dd11be90f12a
diff --git a/packages/NetworkStack/Android.bp b/packages/NetworkStack/Android.bp
index 5f1f26d..f210840 100644
--- a/packages/NetworkStack/Android.bp
+++ b/packages/NetworkStack/Android.bp
@@ -36,6 +36,7 @@
 android_app {
     name: "NetworkStack",
     sdk_version: "system_current",
+    min_sdk_version: "28",
     certificate: "networkstack",
     privileged: true,
     static_libs: [