Clean up out of date logic for starting activity

- ASS#handleNonResizableTaskIfNeeded(), no longer necessary
  to find proper stack for the task on default display since
  exception would be thrown if the task cannot be launched
  on target display.
- ActivityStarter#computeStackFocus(), no more dynamic or
  static stacks.

Bug: 111363427
Test: atest ActivityManagerMultiDisplayTests

Change-Id: Id6da1e1e448b8bc525820cbf407eadd8c86a12bf