Don't recreate Desktop Mode Taskbar if already in Desktop Mode.

The problem: Due to complexitiy of recent animation, taskbar is currently getting recreated upon recent animation being canceled for Desktop Mode.
Upon canceling of recent animation showDesktopApps is called again, but user is already in desktop mode.

The solution: simply check if user is in desktop mode upon getting onEnterDesktopMode transition callback before recreating taskbar.

Test: Manual
Bug: 399857150
Flag: EXEMPT bugfix
Change-Id: I8f36a8c8c834bca7be1ce7164cd712a7578c1ab4