[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, rather than doing that, how about we just change the condition to 
```python
if release.product == 'Firefox for Android' and release.channel == 'Release'
```
The channel page linked to by the get_download_url function will present download buttons for both mobile and desktop.

By that logic, we could also link to https://www.mozilla.org/en-US/firefox/channel/#firefox for the release channel too, which not only presents download buttons for both mobile and desktop, but also exposes users who care enough to read release notes to the fact that we have beta and aurora channels they might be interested in.