DownloadProvider: Remove download notification after preview

Removes download notification from the notification bar after
previewing it. There should be an additional check for download
shown in the notifications after completion to prevent from
situation when ContentObserver is unregistered and the onChange
is never called

Change-Id: I339c08152caf53342cf17e34c46e187fdad5a4c0
Merge "DownloadProvider: Remove download notification after preview" into kitkat