Replace the way to run Intents.init() and Intents.release()

Instead of encouraging Barista users to use something like this to mock the camera

```
try {
Intents.init();
BaristaIntents.mockAndroidCamera();
clickOn(R.id.launch_camera);
} finally {
Intents.release();
}
```

create an easy-to-use Rule that does it.

Related PR: https://github.com/AdevintaSpain/Barista/pull/375/files