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/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
 }