Fix broken transition when caceling enter split screen

Fallback both the following flows to cancel entering split by launching
a solo task to consolidate the cancel transition:
1. not supporting multi-instances split
2. click on the stashed app

Fix: 265736106
Bug: 265898485
Test: repro steps of the bug
Change-Id: I30ca6555dd260722f545538f4594b8b4e12c71d3