Fix a couple of crashes when taskbar isn't yet attached

- Handle back to back addTaskbarIfNecessary(). Specifically, handle
  cleanup() before the previous add was attached. This is already
  handled on WM side, just had to remove isAttachedToWindow() check.
- Add isAttachedToWindow() check to isViewInTaskbar()

Test: call addTaskbarIfNecessary() three times back to back, ensure no crash.

Fixes: 180509238
Bug: 171917176
Change-Id: Id68b6c376029eb5c651f47afa56666391c516aef