Remove "don't show" option from work notifications settings

Option to not show notifications is still available for
device-wide locks screen notification settings.

Also s/On lock screen/When work profile is locked/ for work
notifications since it applies not only to lock screen but to
notification shade as well.

Bug: 64829587
Test: make RunSettingsRoboTests
Change-Id: Ie3ec461f4c8887a5a49cd92b5350ea683f701027