Ring volume icon stuck at normal

Fix the bug where dragging ring volume slider all the way to left
would not trigger an icon change, on devices that do not have
a vibrator.

Solution: offer a boolean config for volumeDownToEnterSilent.

Bug: 265459313

Test: Manual. Slide ring volume to left in settings. Ring volume
icon should switch to mute/vibrate and not get stuck at normal.

Change-Id: I0d5f71a86703eda61c0d831046b2e0c3f5dabb84