[ADF-4364] Fix Back button action on Viewer Component (#4613)

* [ADF-3364] Fix Back button action on Viewer Component

* [ADF-4364] Fix Module providers

* [ADF-4364] Add unit tests

* [ADF-4364] Fix unit tests

* [ADF-4364] Fix lazy loading module of Viewer component

lib/core/services/previous-route.service.ts

@@ -0,0 +1,42 @@
+/*!
+ * @license
+ * Copyright 2019 Alfresco Software, Ltd.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import { Injectable } from '@angular/core';
+import { Router, NavigationEnd } from '@angular/router';
+
+@Injectable({
+    providedIn: 'root'
+})
+export class PreviousRouteService {
+
+    private previousUrl: string;
+    private currentUrl: string;
+
+    constructor(private router: Router) {
+        this.currentUrl = this.router.url;
+        this.router.events.subscribe((event) => {
+            if (event instanceof NavigationEnd) {
+                this.previousUrl = this.currentUrl;
+                this.currentUrl = event.url;
+            }
+        });
+    }
+
+    public getPreviousUrl(): string {
+        return this.previousUrl;
+    }
+}