sink-input: Add volume_writable to pa_sink_input.

sink-input: Add volume_writable to pa_sink_input.

This is pretty cosmetic change; there's no actual functionality added.
Previously the volume_writable information was available through the
pa_sink_input_is_volume_writable() function, but I find it cleaner to have a
real variable.

The sink input introspection variable name was also changed from
read_only_volume to volume_writable for consistency.