Move 'Swap volume buttons' option to 'Buttons' menu

- no one would expect to find a volume buttons option under
  Settings>Display & Lights>Rotation
- other volume buttons options are already under Settings>Buttons
- the feature actually does not depend on 'Auto-rotate screen' option,
  it depends on screen orientation which can change also by other
  means than sensor driven auto rotation - orintation change enforced
  by top app, by dock, by keyboard slide...
- title changed to 'Reorient volume buttons' (to hint at orientation)

Change-Id: I8c03c885c6f2c9ef8d23561b0a3a1a6019512c6e