Update some policy around multi-window windowing mode

- Allow SysUI to start task in always-on-top mode to ensure
  visibility and to allow SysUI to control the visibility using
  the WCT setHidden call.
- Expose unregisterTaskOrganizer call for SysUI
- Skip dispatching status/nav bar insets for always on top mw
  tasks. Since they are effectively floating, it doesn't make
  sense for us to send bar insets
- Fix issue with preferred windowing mode being referenced too
  early (before computeLaunchParams), and apply the windowing
  mode to the root task when it is brought forward

Bug: 151392361
Test: atest WmTests:ActivityOptionsTest
Test: atest InsetsStateControllerTest
Change-Id: Ia3d8a369c902928e8de51a6b7479b10848ebe44a