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

[rocboronat]

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