allow to use mobile network to backup photos

1. mobile network includes 2G/3G/4G.
2. only use WiFi to backup photos by default.
3. always use WiFi when both network types available.
Here we should encapsulate the logic of checking network into a separate function, like this:

```java
private boolean checkNetworkStatus() {
// if has wifi, true
// else if has 3g and allow mobile connection, true
// otherwise, false
}
```