Remove old caption bar before adding a new one

When adding a new window caption, check if old one is present. If it is,
remove it before adding a new one.

Bug: 257121505
Test: manual, open a task in desktop mode, swipe to overview, click on
overview tile, task is again visible, resize task to check that only one
caption bar is shown

Change-Id: Ib4cc3934207f6d51a2ef101e4685285dd3f82743