Add reporting of activity movement for search manager.

This adds a new API with the activity manager to find out about movement between
activities.  For my sanity, the old IActivityWatcher is now renamed to
IActivityController, and the new activity movement interface is named
IActivityWatcher.

This changes the search manager itself to use the new API to manage its state.
Note that there are still problems when going back to the search dialog after
it was hidden -- the suggestions window no longer appears until you explicitly
dismiss and re-show it.