Prevent dismissal of notification.

Fixes #12681

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

@@ -270,6 +270,7 @@ public class KeyCachingService extends Service {
     builder.setSmallIcon(R.drawable.icon_cached);
     builder.setWhen(0);
     builder.setPriority(Notification.PRIORITY_MIN);
+    builder.setOngoing(true);
 
     builder.addAction(R.drawable.ic_menu_lock_dark, getString(R.string.KeyCachingService_lock), buildLockIntent());
     builder.setContentIntent(buildLaunchIntent());