Fix NPE caused by applications that try to hide keyguard while it's not showing

Fixes bug 7466360

Change-Id: Idec4c60c2ca2477e49e0f4526ae2357353d4a1e3

policy/src/com/android/internal/policy/impl/keyguard/KeyguardViewMediator.java

@@ -881,7 +881,9 @@ public class KeyguardViewMediator {
      * Dismiss the keyguard through the security layers.
      */
     public void dismiss() {
-        mKeyguardViewManager.dismiss();
+        if (mShowing && !mHidden) {
+            mKeyguardViewManager.dismiss();
+        }
     }
 
     /**