Add task affiliation

Introduces new flag Intent.FLAG_ACTIVITY_LAUNCH_BEHIND which
causes the newly launched task to affiliate with the launching task.
(Later this flag will also launch the task behind the current task).
This shows up in a new member of the RecentTaskInfo class. This also
causes the recents list returned by getRecentsInfo to be rearranged
so that affiliated tasks are together.

Fixes bug 16157517.

Change-Id: Ia1386af50da2f01809278b62d249f05c6a0de951