Fix issue #16662560: SingleTop activity is getting instantiated multiple time

Introduce a concept of a "root affinity" to a task -- this is the
affinity of the initial activity in the task.  Use this instead of
the current affinity in findTaskLocked(), where we look for an
existing task to use for a NEW_TASK intent.

This changes the semantics of the new "relinquish task identity" mode
so that it doesn't relinquish the root affinity of the task.  This
means when we are in the old style application-based recents matching
of findTaskLocked(), we will never count these tasks as the same as
the application's tasks only because they have relinquished their
identity to that application.  This is probably okay, it is basically
putting a different line between new document-centric recents and
old application-centric recents when they are mixed together.

Change-Id: I73a22ead9bd08e98bf67ad035a017f828c6a6715