Prevent cancel entering split due to animation got canceled

There are chances the entering app transition got canceled by performing
rotation transition. This checks if there is any child task existed in
split screen before fallback to cancel entering flow.

Fix: 264734862
Test: atest WMShellUnitTests
Test: repro steps of the bug
Change-Id: I7647339514b22a03051b62772c01bcec440e2046