Add loading indicator to analytics device dropdown (#299)

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## Description

Added a loading indicator to analytics device dropdown, for a better UX,
and since the occupancy devices api takes longer than the other tabs, it
makes more sense to add this feature.

## Type of Change

<!--- Put an `x` in all the boxes that apply: -->

- [x] ✨ New feature (non-breaking change which adds functionality)
- [ ] 🛠️ Bug fix (non-breaking change which fixes an issue)
- [ ] ❌ Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] 🧹 Code refactor
- [ ] ✅ Build configuration change
- [ ] 📝 Documentation
- [ ] 🗑️ Chore

lib/pages/analytics/models/occupancy_heat_map_model.dart

@@ -19,7 +19,9 @@ class OccupancyHeatMapModel extends Equatable {
       eventDate: DateTime.parse(
         json['event_date'] as String? ?? '${DateTime.now()}',
       ),
-      countTotalPresenceDetected: json['count_total_presence_detected'] as int? ?? 0,
+      countTotalPresenceDetected: num.parse(
+        json['count_total_presence_detected']?.toString() ?? '900',
+      ).toInt(),
     );
   }
 

lib/pages/analytics/modules/air_quality/widgets/aqi_type_dropdown.dart

@@ -6,7 +6,7 @@ enum AqiType {
   aqi('AQI', '', 'aqi'),
   pm25('PM2.5', 'µg/m³', 'pm25'),
   pm10('PM10', 'µg/m³', 'pm10'),
-  hcho('HCHO', 'mg/m³', 'cho2'),
+  hcho('HCHO', 'mg/m³', 'ch2o'),
   tvoc('TVOC', 'mg/m³', 'voc'),
   co2('CO2', 'ppm', 'co2');
 

lib/pages/analytics/modules/energy_management/widgets/analytics_device_dropdown.dart

@@ -28,15 +28,29 @@ class AnalyticsDeviceDropdown extends StatelessWidget {
             ),
           ),
           child: Visibility(
-            visible: state.devices.isNotEmpty,
-            replacement: _buildNoDevicesFound(context),
-            child: _buildDevicesDropdown(context, state),
+            visible: state.status != AnalyticsDevicesStatus.loading,
+            replacement: _buildLoadingIndicator(),
+            child: Visibility(
+              visible: state.devices.isNotEmpty,
+              replacement: _buildNoDevicesFound(context),
+              child: _buildDevicesDropdown(context, state),
+            ),
           ),
         );
       },
     );
   }
 
+  Widget _buildLoadingIndicator() {
+    return const Center(
+      child: SizedBox(
+        width: 24,
+        height: 24,
+        child: CircularProgressIndicator(strokeWidth: 3),
+      ),
+    );
+  }
+
   static const _defaultPadding = EdgeInsetsDirectional.symmetric(
     horizontal: 20,
     vertical: 2,