aconfig: add fixed read only flag

Add a new field in the declaration to indicate whether
the permission can be overridden.

When the field “is_fixed_read_only” is set to true,
the flag permission will be set as fixed “READ_ONLY”,
and the permission should not be changed by Gantry.

Bug: 292521627
Test: atest aconfig.test
Change-Id: Ic9bcd7823bccb8b947cf05568c7ced3763490a23
diff --git a/tools/aconfig/tests/first.values b/tools/aconfig/tests/first.values
index e524404..a450f78 100644
--- a/tools/aconfig/tests/first.values
+++ b/tools/aconfig/tests/first.values
@@ -16,3 +16,9 @@
     state: ENABLED
     permission: READ_WRITE
 }
+flag_value {
+    package: "com.android.aconfig.test"
+    name: "enabled_fixed_ro"
+    state: ENABLED
+    permission: READ_ONLY
+}