Allow a root activity to relinquish task identity

If a an acitivty with attribute android:relinquishTaskIdentity true
is the root activity of a task then the intent of that task will
be that of the first activity in the stack with
android:relinquishTaskIdentity set false.

The ability to set intent also includes the ability to set the
TaskDescription of the task.

Fixes bug 15675610.
Fixes bug 10428661.

Change-Id: Ib28a9eae3b9832eeeef9106adbebe344184ee5ae