Make THEATER_MODE_ON a System API.

Bug: 19043538
Change-Id: Ibf0d6b45868ca90eb792a40c5d1b1395774486b8
diff --git a/core/java/android/provider/Settings.java b/core/java/android/provider/Settings.java
index e055203..dddbe78 100644
--- a/core/java/android/provider/Settings.java
+++ b/core/java/android/provider/Settings.java
@@ -18,6 +18,7 @@
 
 import android.annotation.SdkConstant;
 import android.annotation.SdkConstant.SdkConstantType;
+import android.annotation.SystemApi;
 import android.app.SearchManager;
 import android.app.WallpaperManager;
 import android.content.ComponentName;
@@ -5104,6 +5105,7 @@
          * Whether Theater Mode is on.
          * {@hide}
          */
+        @SystemApi
         public static final String THEATER_MODE_ON = "theater_mode_on";
 
         /**