Showing task selection menus.

These popup menus are shown when a clicked app has > 1 recent tasks.
We still activate the latest task in addition to showing the menu.

Showing task icons in the menu, as well as showing the menu in its UX-polished
form is not yet implemented.

Bug: 20024603
Change-Id: I05e8fe9592501fdd01cbc9032080e4286a658a65