Optionally supress status bar notifications in dnd

If 'allow visual signals -> when the screen is on' is off.

Test: runtest systemui-notification and manual - confirm triggering
dnd hides icons of existing notifications; confirm icons return when dnd
is turned off, verify new icons are hidden when dnd is on, confirm
supressed notification doesn't re-alert when dnd turned off.
Bug: 73455127
Change-Id: If6e5035d6563c70de77604196ea37a16118847a7