sink, source: Use the global configuration for the avoid_resampling default

Previously avoid_resampling was always false unless the sink or source implementation explicitly configured the variable. The null sink doesn't explicitly configure it, so it didn't switch the sample rate as expected when avoid_resampling was enabled.

This change means that also sinks that don't support rate switching can have avoid_resampling set to true, but I think that's fine, because pa_sink_reconfigure() doesn't try to do anything if the reconfigure() callback isn't set.

Fixes: #923 (closed)