OmniGears: Fix removing VolumeWake button when config_show_volumeRockerWake is set to false

If a preference belongs to a preferenceCategory, that category needs to be referenced first
if we're to remove any preference inside it. Here we're using the parent preferenceScreen
to remove mVolumeWake preference while it belongs to volumeCategory. So use volumeCategory
instead of prefScreen.

Change-Id: I0be30bd0c29fcad31d2e4aa83e16692a618b9a52