viewSelector: don't use resetShowAppsButton when pressing escape

Set the checked property explicitly. This is because
resetShowAppsButton() will only be used to flip the button state
blocking the page change in a future commit.

https://bugzilla.gnome.org/show_bug.cgi?id=693974

js/ui/viewSelector.js

@@ -268,7 +268,7 @@ const ViewSelector = new Lang.Class({
             if (this._searchActive)
                 this.reset();
             else if (this._showAppsButton.checked)
-                this._resetShowAppsButton();
+                this._showAppsButton.checked = false;
             else
                 Main.overview.hide();
             return true;