Adding support for loading preference keys as @string resources

Adding support for preference keys as string resources. 

```
<CheckBoxPreference
      android:key="@string/settings_wake_lock_key"
      android:title="..."
      android:summary="..."/>
```

```
<string name="settings_wake_lock_key">wake_lock</string>
```

Currently, ```findPreference("wake_lock")``` would return ```null```. With this patch, it will return the ```CheckBoxPreference```.

I'm not all that familiar with the organization of Robolectric, so let me know if I should move the code or something.