Update THREAD_ENABLED key string in ThreadPersistentSettings

Updated from Thread_enabled to thread_enabled

Test: presubmit
Change-Id: If0296f4d4bdf37f4564ee6fabdeffffe9d3020dd
diff --git a/thread/service/java/com/android/server/thread/ThreadPersistentSettings.java b/thread/service/java/com/android/server/thread/ThreadPersistentSettings.java
index 5cb53fe..923f002 100644
--- a/thread/service/java/com/android/server/thread/ThreadPersistentSettings.java
+++ b/thread/service/java/com/android/server/thread/ThreadPersistentSettings.java
@@ -61,7 +61,7 @@
 
     /******** Thread persistent setting keys ***************/
     /** Stores the Thread feature toggle state, true for enabled and false for disabled. */
-    public static final Key<Boolean> THREAD_ENABLED = new Key<>("Thread_enabled", true);
+    public static final Key<Boolean> THREAD_ENABLED = new Key<>("thread_enabled", true);
 
     /******** Thread persistent setting keys ***************/