Add StageOrderOperator to keep track of stages indicies

* Brings parity with everything except double tap to swap
for using StageOrderOperator instead of directly using
mMainStage and mSideStage

Bug: 349828130
Flag: com.android.wm.shell.enable_flexible_split
Change-Id: I660c1636601381f324e5978b18d2c207dece7579