Always allow toasts from foreground apps

If the app is in foreground, allow the app to post
toasts even when notifications are blocked for that app.

Also added tests for when Toasts should/shouldn't show.

Test: atest frameworks/base/services/tests/uiservicestests/src/com/android/server/notification/NotificationManagerServiceTest.java
Change-Id: I4f14b313d9b751c5ad00d309462aee6460cddad0
Fixes: 71546399