Add taskbar stashing feedforward, i.e. hint at upcoming stash/unstash

Also fix unstashing not working in 3P launchers by moving the unstash call from LauncherTaskbarUIController to TaskbarActivityContext

Test: long press taskbar background and stashed handle, watch it hint towards the new stashed/unstashed state respectively and then complete the animation when the long press is triggered
Fixes: 193926311
Fixes: 192926350
Change-Id: I0e538be9129bf5c600d07f360b8106d7077862ad