vaapipostproc: Use level value for skin-tone-enhancement filter.

Currently the parameter of skin-tone-enhancement filter is forced to zero. In fact it could be set different value by the user. So create a new property named as "skin-tone-enhancement-level" for accepting the used defined parameter value. At the same time, skin-tone-enhancement is marked as deprecated. When skin-tone-enhancement-level is set, skin-tone-enhancement will be ignored.