google-play-ads: don't show drawer ad unless on android and not in googl...

...e play
Huh? What's wrong?

If "shouldShowAds should check the experiments", the code would be like this:

```
 if GooglePlayAdService.shouldShowAds()
      @GooglePlayAdDrawer = if params.googlePlayDrawer is 'install-button' \
                            then new GooglePlayAdDrawerInstallButton()
                            else new GooglePlayAdDrawerControl()
```

but we'd still have to hit `getExperiments` to grab the experiment param