Allow direct installation of downloaded APKs

[Fs00]

This PR adds a required permission to the manifest so that downloaded APKs can be installed directly by tapping on the download notification on Android 6+.

I've also extracted download-related methods that were in UiUtils in a separate DownloadUtils class (I felt they were out of place while looking at the code) and renamed the receiver responsibile for intercepting clicks on pending download notifications.

Fixes #1067

[Fs00]

@maniac103 I see this has been parked for quite some time, are there any doubts/uncertainties around the proposed changes?

[maniac103]

Uh, I completely forgot about this. I'd prefer to keep the class renaming out, or at least keep the old name for EventReceiver since the intention is to keep it reusable for other purposes.

[Fs00]

or at least keep the old name for EventReceiver since the intention is to keep it reusable for other purposes.

Removed the change, let me know if there's anything else