[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.
I don't think the check for `release is not None` is very useful here: if `release` is None, an `AttributeError` will be raised when we try to access the `channel` attribute in line 517 anyway.