Fix minor a11y issues in field filter

aria-expanded is a true/false/undefined state so we need to make sure
even if showFilter is undefined, we will print false (that's why the
double negation)

aria-haspopup doesn't really fit, since this is not a popup in the sense
the ARIA standard describes ("A popup is generally presented visually as
a group of items that appears to be on top of the main page content.")

Add aria-controls to reference the controlled element.

Fix elastic#12638 and toggle label when filter toggles

src/core_plugins/kibana/public/discover/components/field_chooser/field_chooser.html

@@ -54,16 +54,16 @@ <h3 class="sidebar-list-header-label" id="available_fields" tabindex="0">Availab
         ng-class="{ 'kuiButton--basic': !filter.active, 'kuiButton--primary': filter.active, 'hidden-xs': !showFields, 'hidden-sm': !showFields }"
         class="kuiButton kuiButton--small pull-right discover-field-filter-toggle"
         ng-click="showFilter = !showFilter"
-        aria-label="Show field settings"
-        aria-haspopup="true"
-        aria-expanded="{{showFilter}}"
+        aria-label="{{showFilter ? 'Hide' : 'Show'}} field settings"
+        aria-expanded="{{!!showFilter}}"
+        aria-controls="discover-field-filter"
       >
         <span aria-hidden="true" class="kuiIcon fa-gear"></span>
       </button>
     </h3>
   </div>
 
-  <div class="sidebar-item discover-field-details" ng-show="showFilter">
+  <div class="sidebar-item discover-field-details" ng-show="showFilter" id="discover-field-filter">
     <form role="form">
       <div class="form-group">
         <label for="discoverFieldChooserFilterAggregatable">