Settings: Fix fallback summary for power_button_instantly_locks preference

* Using summary_placeholder instead of summary_empty
  leaves an ugly empty space.

Change-Id: I73a90f1b3203d6cb17e1875dc8163eb67140de89

src/com/android/settings/security/screenlock/PowerButtonInstantLockPreferenceController.java

@@ -78,7 +78,7 @@ public class PowerButtonInstantLockPreferenceController extends AbstractPreferen
                     R.string.lockpattern_settings_power_button_instantly_locks_summary,
                     trustAgentLabel));
         } else {
-            preference.setSummary(R.string.summary_placeholder);
+            preference.setSummary(R.string.summary_empty);
         }
     }