[fix bug 1061715] Change download_url if it is an Android release

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1061715

Reference page: https://www.mozilla.org/en-US/mobile/32.0/releasenotes/

When toggling "View Notes To" in the "Mobile" mode, the "Download" link should point to the Google Play URL.
Actually, the section I linked to isn't the whole story. There's also some logic in [bedrock.mozorg.helpers.download_buttons](https://github.com/mozilla/bedrock/blob/master/bedrock/mozorg/helpers/download_buttons.py#L223-L247) that we'd need to incorporate to generate the correct link.