fix: Disconnect from room before app closes. (#218)

* fix: Disconnect from room before app closes.

* chore: add app state listener.

* Remove unused import.

* fix `flutter test`.

* singleton.

example/pubspec.lock

@@ -226,6 +226,13 @@ packages:
       url: "https://pub.dartlang.org"
     source: hosted
     version: "0.9.17"
+  flutter_window_close:
+    dependency: "direct main"
+    description:
+      name: flutter_window_close
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "0.2.2"
   google_fonts:
     dependency: "direct main"
     description:
@@ -351,7 +358,7 @@ packages:
       name: path_provider_android
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "2.0.21"
+    version: "2.0.22"
   path_provider_ios:
     dependency: transitive
     description:
@@ -552,7 +559,7 @@ packages:
       name: uuid
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "3.0.6"
+    version: "3.0.7"
   vector_math:
     dependency: transitive
     description:
@@ -573,7 +580,7 @@ packages:
       name: win32
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "3.1.1"
+    version: "3.1.2"
   xdg_directories:
     dependency: transitive
     description: