Recents backup helper

Handles backup/restore of recent tasks for the system.  Currently the
thumbnails are not saved.

At restore time the historical task records are placed in a designated
separate location rather than directly in the live bookkeeping; this
avoids ID duplication issues and makes it easier to deal with lazy
adoption of the historical task state as apps are installed on the
device post-restore.

Bug 17303286
Bug 15986349

Change-Id: Ie156c1e2ab9c9a7e7ac0447b27016fdcef55dded