[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.
If the user is looking at release notes for an aurora or beta channel release, the download link should probably take that into account instead of always going to the release channel. We should be able to use https://github.com/mozilla/bedrock/blob/master/bedrock/mozorg/helpers/download_buttons.py#L81-L127 for this.