Allow more transient icons in app shelf

* Build the icon views dynamically rather than having a fixed number
  in the layout XML file
* Increase the maximum number of icons
* Keep the sort order stable as icons are added
* Infer when icons should be added and removed by observing recent
  tasks.

Bug: 22031100
Change-Id: Ifa78695ab8b332a47ff116bb502d8d095881e3f0