Integrate launch-to-side split transition to shell transition

Integrate launch-to-side split transition to shell transition.
Because there is no guarantee for launching activity to split
successfully before actually launching it, trigger enter split
transition after observed new task launching into split.

Bug: 206487881
Test: atest SplitTransitionTests
Test: drag to enter split with shell transition works
Change-Id: Idda7c3b02689753cea1a1fe80264145f1a449f73