[Thread] make Thread flags fixed_read_only

The newly added Thread flags (i.e. configuration_enabled and
channel_max_power_enabled) are not visible on gantry. This is due to the
flags are not fixed.

Bug: 346686506
Change-Id: I4c50507c90057132aa336a401a53e9c560acd8bc
diff --git a/common/thread_flags.aconfig b/common/thread_flags.aconfig
index a3f77a7..c11c6c0 100644
--- a/common/thread_flags.aconfig
+++ b/common/thread_flags.aconfig
@@ -12,6 +12,7 @@
 flag {
     name: "configuration_enabled"
     is_exported: true
+    is_fixed_read_only: true
     namespace: "thread_network"
     description: "Controls whether the Android Thread configuration is enabled"
     bug: "342519412"
@@ -20,6 +21,7 @@
 flag {
     name: "channel_max_powers_enabled"
     is_exported: true
+    is_fixed_read_only: true
     namespace: "thread_network"
     description: "Controls whether the Android Thread setting max power of channel feature is enabled"
     bug: "346686506"