Specify is_fixed_read_only: true for net security mainline trunk stable flag

Mainline trunk stable flag must specify is_fixed_read_only: true since
server-side flags not supported for mainline in the trunk stable infrastructure
as of now.

Change-Id: Ibb2d599ee655f871f08b36bbc3cd6e286cf0d3c8
Test: TH
diff --git a/common/OWNERS b/common/OWNERS
index e7f5d11..989d286 100644
--- a/common/OWNERS
+++ b/common/OWNERS
@@ -1 +1,2 @@
 per-file thread_flags.aconfig = file:platform/packages/modules/Connectivity:main:/thread/OWNERS
+per-file networksecurity_flags.aconfig = file:platform/packages/modules/Connectivity:main:/networksecurity/OWNERS
\ No newline at end of file
diff --git a/common/networksecurity_flags.aconfig b/common/networksecurity_flags.aconfig
index ef8ffcd..6438ba4 100644
--- a/common/networksecurity_flags.aconfig
+++ b/common/networksecurity_flags.aconfig
@@ -6,4 +6,5 @@
     namespace: "network_security"
     description: "Enable service for certificate transparency log list data"
     bug: "319829948"
+    is_fixed_read_only: true
 }