Improve dismiss split transition

Improve this transition by seperating it as 2 transitions:
1. Only dismiss invisible split side and extand another one side
   root task as full screen bounds.
2. After first transition finished, fade-out the decor surface
   and reparent the child task to TDA top.

Bug: 200245358
Test: manual
Change-Id: I1a0c4d1715765e7de0ce2628e978cab392d2b07e