ActionHandler stage 2.

Extract interface, dropping any Activity typed param.
Introduce AbstractActionHandler, depends *only* on concrete Activity.
Make only subclasses know about ManageActivity and PickActivity.
Distribute functionality accordingly.
Pull openInNewWindow into ActivityHandler...
    update different cases to all send a stac....
    and update launching code to handle a stack with just a root.
Add an almost pointless test for openInNewWindow (but the test harness is in place).

Change-Id: I729ee0c06d3de0c05c0bea118533dd1c1f274783