update dirents or repos data automatically

fix add background auto-update per 5 mins (no UI refresh) #179
The logic is better now, but it can still be improved, like this:

```java
forceRefresh = forceRefresh || isReposRefreshTimeOut();
if (!Utils.isNetworkOn() || !forceRefresh) {
    List<SeafRepo> repos = getDataManager().getReposFromCache();
    if (repos != null) {
        updateAdapterWithRepos(repos);
        return;
    }
}
ConcurrentAsyncTask.execute(new LoadTask(getDataManager()));
```
The same for `navToDirectory`.