back button clears selection: room list and chat

When a room or event is selected and the Android back button is pressed,
don't pop but clear the current selection. Fixes #399.

lib/pages/views/chat_list_view.dart

@@ -26,7 +26,15 @@ class ChatListView extends StatelessWidget {
               : controller.selectedRoomIds.isEmpty
                   ? SelectMode.normal
                   : SelectMode.select;
-          return Scaffold(
+          return VWidgetGuard(
+            onSystemPop: (redirector) async {
+                if (controller.selectedRoomIds.isNotEmpty) {
+                    controller.cancelAction();
+                    redirector.stopRedirection();
+                }
+            },
+            child:
+          Scaffold(
             appBar: AppBar(
               elevation: MediaQuery.of(context).size.width >
                       FluffyThemes.columnWidth * 2
@@ -231,7 +239,7 @@ class ChatListView extends StatelessWidget {
                     child: Icon(CupertinoIcons.chat_bubble),
                   )
                 : null,
-          );
+          ));
         });
   }
 }

lib/pages/views/chat_view.dart

@@ -54,7 +54,15 @@ class ChatView extends StatelessWidget {
           context: context, future: () => controller.room.join());
     }
 
-    return Scaffold(
+    return VWidgetGuard(
+      onSystemPop: (redirector) async {
+        if (controller.selectedEvents.isNotEmpty) {
+          controller.clearSelectedEvents();
+          redirector.stopRedirection();
+        }
+      },
+      child:
+            Scaffold(
       appBar: AppBar(
         leading: controller.selectMode
             ? IconButton(
@@ -694,7 +702,7 @@ class ChatView extends StatelessWidget {
           ),
         ],
       ),
-    );
+    ));
   }
 }