Declare RingVolumePreferenceController xml exempt

The controller is no longer in use in SoundSettings and should not
affect code inspection.

Bug: b/289967737
Test: 1. Remove @Ignore tag from `CodeInspectionTest#runSliceControllerInXmlInspection`
      2. atest CodeInspectionTest
Change-Id: I07cea3c5e2f5282950a63945181cc4711b937bc5