SearchService: improve enabled lookup performance

I didn't benchmark it, but this should be much faster for two reasons:

1. We only ask the PluginService for the Searcher's PluginInfo when
   an already-persisted value is not found in the PrefService.
2. We use the getPlugin(Class pluginClass, Class pluginType) method,
   which is more performant because it needs to dig through only the
   list of plugins of the given type. Previously, we were using
   getPlugin(Class pluginClass) which needed to dig through every
   PluginInfo on every request.

Author: ctrueden

src/main/java/org/scijava/search/DefaultSearchService.java

@@ -32,6 +32,7 @@
 import org.scijava.plugin.AbstractSingletonService;
 import org.scijava.plugin.Parameter;
 import org.scijava.plugin.Plugin;
+import org.scijava.plugin.PluginInfo;
 import org.scijava.prefs.PrefService;
 import org.scijava.service.Service;
 
@@ -50,9 +51,12 @@ public class DefaultSearchService extends
 
 	@Override
 	public boolean enabled(final Searcher s) {
-		final boolean defaultValue = //
-			pluginService().getPlugin(s.getClass()).isEnabled();
-		return prefService.getBoolean(s.getClass(), "enabled", defaultValue);
+		final String enabled = prefService.get(s.getClass(), "enabled");
+		if (enabled != null) return Boolean.valueOf(enabled);
+		// Get the default value from enabled property of PluginInfo.
+		final PluginInfo<Searcher> info = //
+			pluginService().getPlugin(s.getClass(), Searcher.class);
+		return info == null ? false : info.isEnabled();
 	}
 
 	@Override