Only support manual stashing while in app

- For taskbar in overview, don't allow stashing

Fix: 253042515
Test: With taskbar in overview, long press do nothing. Taskbar is still stashable in app
Test: Without taskbar in overview, taskbar still transform to nav handle in overview
Change-Id: I8f353a473a8570df72acda2338c4b984a69dc5c0