Add ability to disable launch-adjacent for a specific root task

- There are some scenarios in which we need to disable launch adjacent
  handling due apps incorrectly trampolining from one launch-adjacent
  task immediately to another launch-adjcent task (thereby clobbering
  the original task).

  eg. User launches A fullscreen -> A
      A launches trampoline B with LAUNCH_ADJACENT -> A | B
      B launches final target C but also with LAUNCH_ADJACENT -> C | B
      but then trampoline B finishes itself -> C

  We attempt to work around this by disabling launch-adjacent for the
  split root task and its children.
- This can also be useful for desktop mode in which we want to disable
  launch adjacent from accidentally launching into the adjacent root
  task specified by SplitScreen (we do this currently by temporarily
  disabling the launch adjacent root), but when we have multi-display
  we may just want to disable launch adjacent specifically for all
  tasks while they are in desktop mode.

Bug: 344216031
Change-Id: Ib8f98c17a7ac9834684968efe4688daeace4e13c
Test: atest ActivityStarterTests
Flag: EXEMPT bugfix