Stop double notifying a11y about Toasts.

TalkBack was speaking all Toasts twice because we were
sending both NOTIFICATION and a WINDOW_STATE_CHANGE events
when Toasts were displayed.

Stop sending the WINDOW_STATE_CHANGE event for toasts.
Bug: 30570188
Change-Id: I26c6dc50350dfd4daf2a706b94fadcf5d1e03557