add some more info to the deploy dialog

UX changes to the deploy dialog:

- have an initial focused item, so that escape will close the dialog
- fix a css issue with the radio buttons
- add some helper text to tell the user how to get the ChromeAPT APK
- (misc style nits in `park_polymer.dart`)

Before:
![screen shot 2014-03-16 at 9 50 09 am](https://f.cloud.github.com/assets/1269969/2431466/399725b0-ad30-11e3-9a53-88cfae61ad0d.png)

After:
![screen shot 2014-03-16 at 9 48 28 am](https://f.cloud.github.com/assets/1269969/2431468/44ede598-ad30-11e3-83f5-83b6229ee598.png)

@shepheb