Keep splitting tasks in the background with shell transition

Align with legacy transition to not breaks splitting tasks in the
background and set the flag to support relaunching background splitting
tasks to fullscreen.
Also fix not collecting move_to_front transition changes when
reparenting an existing task to top.

Bug: 275664132
Fix: 280523948
Test: atest WMShellUnitTests WMShellFlickerTests
Test: having a fullscreen app occludings a split pair, launching one of
      the splitting task breaks the split and brings up in fullscreen
Test: going from a split pair to home, launching one of the splitting
      task breaks the split and brings up in fullscreen
Test: having a task supporting auto-pip in split, going to home breaks
      the split pair and entering pip
Change-Id: I134c8c4ecdfe3ee29df9de7b32cfa501c88510c6