Fix not able to put app in split after removed task from side stage

Relies on reorder operation instead of setHidden operation to control
visibility of side stage. When removing task from side stage, keep side
stage visible if there's task remaining on side stage.

Fix: 180631194
Test: atest WMShellUnitTests
Test: manual check drop to split works after removeFromSideStage
Change-Id: I5a92bf33d1d447d4c4e28bb97e3e337927ffb875