Fix issue #16400873: Apps that show ads as new tasks stay in recents...

...even when I hit back or close the activity in UI

Change the semantics of NEW_DOCUMENT to have the recents entry be
removed by default when its activity is finished, with various ways
to explicitly turn off this behavior.

Change-Id: Idc717706d27de80f28b53ad76f9e375d85118d71