[sink-input-ext] Set volume factor in SINK_INPUT_FIXATE.

Previously volume factor was set in SINK_INPUT_PUT and at that time
sink-input was already attached to sink's IO-thread. This means some
data was already passed when the volume factor was applied.
As a fix add volume factor a bit earlier, in
PA_CORE_HOOK_SINK_INPUT_FIXATE.
[policy] Fix const in various places.

Const was casted away from constant strings in multiple places. Also
some places used char* when the string wouldn't be or shouldn't be
changed in the function. Add const to these places and modify functions
so that constant strings are returned as const.