Revert "Revert "Add a way to stash/unstash transient taskbar.""

This reverts commit afc3bff10b74c5fbfa1bbc9da01149032b36b6a5.

Reason for revert: fixing launcher tests broken by original change

We'll only add flags for transient taskbar case

Bug: 256988243
Bug: 256987492
Change-Id: I8915ef5345816316bfb71efc9fc667e4e8db824c