Block custom toasts when app is in the background.

We're blocking a toast in a specific case when the app posts it while
it's in the foreground, but then the toast is about to be shown only
after the app has moved into the background.

Test: atest NotificationManagerServiceTest
Bug: 170571276
Change-Id: Ibfbb636c4857f75f597e27ea0afb31cfa431a368