SoundSettings: Prevent NPE due to linked volumes change

In AOSP, mRingOrNotificationPreference can never be null.

However, ever since the linked notification volumes change
in commit 6430dbd2, there is a
possibility that mRingOrNotificationPreference is null,
causing an NPE when calling showIcon() on it.

Add a null check to prevent that.

Change-Id: I8d7231f50fc1f04acdc706e2d2c775bed7803fca