Fix variable name clash.

* VuFind/Search/Listener/MultiIndexListener.php (getFields): Fix
  variable name clash.
Change MultiIndexListener implementation

* VuFind/Search/Listener/MultiIndexListener.php ($available):
  Removed. Replaced by $shards.
  ($shards): New property. Available shards.
  ($active): Removed.
  (__construct): Set new property.
  (setActiveShards): Removed.
  (onSearchPre): Read selected shards from shards parameter.
  (getFields): Use shard URI to check for strip fields.

Picks up the selected shards from the `shards' search parameter.