Use a PopupWindow to show tooltips

This allows tooltips to work even in a context where they don't belong
in any activity (and therefore no window token to use). It also
simplifies a tiny bit the logic of how to get the view to show up.

Test: Checked tooltip behavior in and outside an app
Bug: 62065980
Change-Id: I6c02009c4fdd6d4bc4fa2cf8019955360506f0ee