Add rollback/observer_rollback_availability_in_millis in allowlist.

This would be needed for testing rollback in CTS and manual testing.

Bug: 416259905
Flag: EXEMPT add allowlist flag
Test: presubmit
Change-Id: I192f219553ce7a2e8e2424f57a1562dab71e1310
diff --git a/framework/java/android/provider/WritableFlags.java b/framework/java/android/provider/WritableFlags.java
index 57bd51e..ad5dafb 100644
--- a/framework/java/android/provider/WritableFlags.java
+++ b/framework/java/android/provider/WritableFlags.java
@@ -544,6 +544,7 @@
                 "rollback_boot/rollback_lifetime_in_millis",
                 "rollback/containing",
                 "rollback/enable_rollback_timeout",
+                "rollback/observer_rollback_availability_in_millis",
                 "rollback/watchdog_explicit_health_check_enabled",
                 "rollback/watchdog_request_timeout_millis",
                 "rollback/watchdog_trigger_failure_count",