Don't stash immediately when drag was canceled

Instead, keep the taskbar open for the normal timeout.

Test: Drag an app from taskbar and drop back in taskbar, ensure taskbar
stays open until 5 second timeout; Drag an app from taskbar and drop
into split screen, ensure taskbar stashes immediately
Flag: ENABLE_TRANSIENT_TASKBAR true
Fixes: 275019813

Change-Id: I06ca0eccbfe313dbae8f6d96c7c09bb659f6a27b
Merged-In: I06ca0eccbfe313dbae8f6d96c7c09bb659f6a27b