A11y - improve announcing

When an alert triggers announcing on one page, then after user
navigate to different page, alert is repeated again.

This change set after new page announce to empty.

Change-Id: If264d14e1cf76a4ba1ba1133430766d9e6e1d9cf
diff --git a/polygerrit-ui/app/elements/core/gr-error-manager/gr-error-manager.ts b/polygerrit-ui/app/elements/core/gr-error-manager/gr-error-manager.ts
index 4f3d7ce6..e4e4056 100644
--- a/polygerrit-ui/app/elements/core/gr-error-manager/gr-error-manager.ts
+++ b/polygerrit-ui/app/elements/core/gr-error-manager/gr-error-manager.ts
@@ -336,7 +336,7 @@
     const el = this._createToastAlert();
     el.show(text, actionText, actionCallback);
     this._alertElement = el;
-    this.fire('iron-announce', {text}, {bubbles: true});
+    this.fire('iron-announce', {text: `Alert: ${text}`}, {bubbles: true});
     this.reporting.reportInteraction('show-alert', {text});
   }
 
diff --git a/polygerrit-ui/app/elements/gr-app-element.ts b/polygerrit-ui/app/elements/gr-app-element.ts
index 2915acd..617034b 100644
--- a/polygerrit-ui/app/elements/gr-app-element.ts
+++ b/polygerrit-ui/app/elements/gr-app-element.ts
@@ -492,6 +492,9 @@
         registrationOverlay.refit();
       });
     }
+    // To fix bug announce read after each new view, we reset announce with
+    // empty space
+    this.fire('iron-announce', {text: ' '}, {bubbles: true});
   }
 
   _handleShortcutTriggered(event: ShortcutTriggeredEvent) {