Let's SimpleAdapter's context be Activity, not Application.

`DownloadsAdapter` will inflate layout, and layout should be under
`Activity`. Actually, current `XposedApp.getInstance()` is an
`Application` context, which doesn't apply custom theme. (Back-ported
version need custom theme).

For more question of context, I think this article describes clearly:
http://www.doubleencore.com/2013/06/context/
Merge pull request #85 from liudongmiao/patch

Let DownloadsAdapter's context be Activity, not Application.