Recents: allow apps to be excluded from removing all apps

Long pressing a recents entry allows to exclude an app from
being killed when pressing the remove all button.

Manually swiping away will still be possible.

Change-Id: Ic6105dc0208a21f587186e3a173cdd78b3c6ef34
Signed-off-by: Dirk Rettschlag <dirk.rettschlag@gmail.com>