Fix performance issue with large number of notifications.

Constructing the notification would call KeyCachingService#isLocked()
many times in a loop. This call is slow, because when the device isn't
locked, it would construct the master secret each time, which can take
50+ ms. Do that twice in a loop a hundred times, and it adds up.

This simplified #isLocked to avoid the unnecessary master secret
generation.

src/org/thoughtcrime/securesms/service/KeyCachingService.java

@@ -79,7 +79,7 @@ public class KeyCachingService extends Service {
   public KeyCachingService() {}
 
   public static synchronized boolean isLocked(Context context) {
-    return getMasterSecret(context) == null;
+    return masterSecret == null && (!TextSecurePreferences.isPasswordDisabled(context) || TextSecurePreferences.isScreenLockEnabled(context));
   }
 
   public static synchronized @Nullable MasterSecret getMasterSecret(Context context) {