Fix app center active tab

Closes PLAT-3907

Test Plan:
- Verify clicking on the app center filter tabs ("All", "Not Installed",
  etc.) navigates to the correct tab.
- Verify the active tab has the active style applied to it.

Change-Id: If002637d598d6c4d07a31079c646e49bd249845b
Reviewed-on: https://gerrit.instructure.com/169640
Reviewed-by: Marc Phillips <mphillips@instructure.com>
Tested-by: Jenkins
QA-Review: Marc Phillips <mphillips@instructure.com>
Product-Review: Weston Dransfield <wdransfield@instructure.com>

app/jsx/external_apps/components/AppFilters.js

@@ -55,7 +55,7 @@ export default class AppFilters extends React.Component {
   }
 
   render() {
-    const activeFilter = this.state.filter || 'all'
+    const activeFilter = store.getState().filter || 'all'
     return (
       <div className="AppFilters">
         <div className="content-box">