Remove toast window immediately

To address a security issue where a toast window can be
used by an app to overlay other apps without a permission
we now allow legacy apps to be able to put at most one
toast window on the screen to prevent adding the same
window over and over again to go around the new restriction
that toast windows are always removed after a timeout.
This change ensures that Toast removes its window immediately.

bug:31340854

Change-Id: Ia7f90844eb64b583321103d090e4407038b41547