Track window position with task position.

Windows were previously ordered by TaskStack/ActivityStack order. This
change provides a data structure in DisplayContent that tracks task
movement. Previously Recents and Home activity windows were always
adjacent because they were on the same stack. With this change windows
from other activities can be placed between the two.

Fixes bug 11338594.

Change-Id: Ie34443ff22f330d015141d97db79370c54920d28