Revert "SplitScrn: Check for all children visibility when updating mVisible."

This reverts commit e984551f.

Flag: EXEMPT revert
Bug: 383444878
Test: atest WMShellUnitTests
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:53367017c1bc1911609f457f1e7d389db9d9f749)
Merged-In: I634dbd58e8f40425c8b648e2aef34a2dca9c85bd
Change-Id: I634dbd58e8f40425c8b648e2aef34a2dca9c85bd