Add flag extend_vb_chain_to_updated_apk as read-only

Due to a limitation in the current flag infra (b/299647580; device
config not yet initialized for the code to be flagged in package
manager), the only way forward is to make the flag read-only. It
effectively turns the flag into a build-time flag, and remains to be off
until Gantry ramp-up.

Bug: 277344944
Test: Use the flag in package manager, it doesn't crash in the flag infra
Change-Id: I059980ca0e15c24b54c3fece7bc69a8301246c7a
diff --git a/core/java/android/security/flags.aconfig b/core/java/android/security/flags.aconfig
index cfc6f48..800149c 100644
--- a/core/java/android/security/flags.aconfig
+++ b/core/java/android/security/flags.aconfig
@@ -20,3 +20,11 @@
     description: "Feature flag for deprecating .fsv_sig"
     bug: "277916185"
 }
+
+flag {
+    name: "extend_vb_chain_to_updated_apk"
+    namespace: "hardware_backed_security"
+    description: "Use v4 signature and fs-verity to chain verification of allowlisted APKs to Verified Boot"
+    bug: "277916185"
+    is_fixed_read_only: true
+}