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: I0914a680f08eb96949b56e08719eb28d2e80ccc9
DownloadProvider: Add transfer speeds in notification

This will show transfer speed in notification when in expanded mode.
For multiple files, the combined speed are displayed.

Screenshot: http://i.imgur.com/xqLRvMo.png

PS1: Initial commit
PS2: clean code
     add missing declarations & import

Change-Id: I801dbe61c7ee59d0c1d14d5851ad6dc3a7678499