Don't start search from a dialog if there is no activity associated with

the search manager at that point.

Still works:
- hitting search when a managed dialog of an app is showing will dismiss
  the dialog and start in-app search (if it supports it), falling back
  on global search
- hitting search when a dialog of an app (not managed by the activity) will
  dismiss the dialog and start global search
- hitting search when a system dialog is showing will just dismiss the dialog.