Implementing app-centric Shelf.

We finally get a UX decision for the Shelf.
Shelf won't have 2 parts - pinned and recents; it will have only 1 list where
all pinned and recent activities of the same kind are grouped under a single icon.
Clicking at that icon activates the latest running activity if it's present, or
starts a new one otherwise.

The above part is implemented in this CL. I removed stuff related to the separate
Recents pane, and moved surviving code to NavigationBarApps.

Later, we'll have menus popping up from the icon, which will allow pinning and unpinning,
and choosing a concrete activity to activate.

Bug: 20024603
Change-Id: Ia08fe62939f92c7ee4f102c07a31e7168a11f010