Show prelogin only once

 Bug: 5081641

Change-Id: I1c625e3528de9a7cd37dae5b8b428ff141759e14
diff --git a/src/com/android/browser/PreferenceKeys.java b/src/com/android/browser/PreferenceKeys.java
index 4174732..ce8ec9f 100644
--- a/src/com/android/browser/PreferenceKeys.java
+++ b/src/com/android/browser/PreferenceKeys.java
@@ -70,6 +70,7 @@
     static final String PREF_NORMAL_LAYOUT = "normal_layout";
     static final String PREF_SMALL_SCREEN = "small_screen";
     static final String PREF_WIDE_VIEWPORT = "wide_viewport";
+    static final String PREF_RESET_PRELOGIN = "reset_prelogin";
 
     // ----------------------
     // Keys for lab_preferences.xml