Prevent taskbar on phone from crashing

* Update code behind flag to work with transient taskbar
* We explicitly set the mDestroyed flag to false in
TaskbarActivityContext because it gets set to true when
the flag is on, but nothing resets it unless it gets
recreated normally.

Bug: 274517647
Flag: persist.wm.debug.hide_navbar_window
Test: Enable flag and verify no crash
Change-Id: I9a149ffb47a10efedfc9dff8399d12d4c1a31553