Fix some issues with notif blocking in settings

- Fix cast cast exception
- Make sure unblocking a notification channel also actually
unblocks it

Test: make -j64 RunSettingsRoboTests ROBOTEST_FILTER="com.android.settings.notification"
Fixes: 177968301
Fixes: 177907827
Change-Id: Ibad56e102e167bcf004fbd0f9873cc29834a30c1