Adds silence toggle to notification settings.

This change modifies the UI that appears when a notification is
long-pressed to include a third option when the notification could
potentially buzz or make sound, allowing the user to promote or demote
the notification's channel such that it will not alert (or will start
alerting) for future notifications.

Test: atest SystemUITests
Bug: 116622974
Change-Id: Ia070d0ef1b181b4de8b3d49ace9ff2a65fed8deb