cherry pick available cameras into new implementation

@@ -264,6 +264,7 @@ class MethodChannelMobileScanner extends MobileScannerPlatform {
 
     _textureId = textureId;
 
+    final int? numberOfCameras = startResult['numberOfCameras'] as int?;
     final bool hasTorch = startResult['torchable'] as bool? ?? false;
 
     final Map<Object?, Object?>? sizeInfo =
@@ -279,7 +280,11 @@ class MethodChannelMobileScanner extends MobileScannerPlatform {
       size = Size(width, height);
     }
 
-    return MobileScannerViewAttributes(hasTorch: hasTorch, size: size);
+    return MobileScannerViewAttributes(
+      hasTorch: hasTorch,
+      numberOfCameras: numberOfCameras,
+      size: size,
+    );
   }
 
   @override

@@ -239,6 +239,7 @@ class MobileScannerController extends ValueNotifier<MobileScannerState> {
       );
 
       value = value.copyWith(
+        availableCameras: viewAttributes.numberOfCameras,
         cameraDirection: effectiveDirection,
         isInitialized: true,
         isRunning: true,

@@ -4,12 +4,16 @@ import 'dart:ui';
 class MobileScannerViewAttributes {
   const MobileScannerViewAttributes({
     required this.hasTorch,
+    this.numberOfCameras,
     required this.size,
   });
 
   /// Whether the current active camera has a torch.
   final bool hasTorch;
 
+  /// The number of available cameras.
+  final int? numberOfCameras;
+
   /// The size of the camera output.
   final Size size;
 }

@@ -8,6 +8,7 @@ import 'package:mobile_scanner/src/mobile_scanner_exception.dart';
 class MobileScannerState {
   /// Create a new [MobileScannerState] instance.
   const MobileScannerState({
+    required this.availableCameras,
     required this.cameraDirection,
     required this.isInitialized,
     required this.isRunning,
@@ -20,6 +21,7 @@ class MobileScannerState {
   /// Create a new [MobileScannerState] instance that is uninitialized.
   const MobileScannerState.uninitialized(CameraFacing facing)
       : this(
+          availableCameras: null,
           cameraDirection: facing,
           isInitialized: false,
           isRunning: false,
@@ -28,6 +30,11 @@ class MobileScannerState {
           zoomScale: 1.0,
         );
 
+  /// The number of available cameras.
+  ///
+  /// This is null if the number of cameras is unknown.
+  final int? availableCameras;
+
   /// The facing direction of the camera.
   final CameraFacing cameraDirection;
 
@@ -55,6 +62,7 @@ class MobileScannerState {
 
   /// Create a copy of this state with the given parameters.
   MobileScannerState copyWith({
+    int? availableCameras,
     CameraFacing? cameraDirection,
     MobileScannerException? error,
     bool? isInitialized,
@@ -64,6 +72,7 @@ class MobileScannerState {
     double? zoomScale,
   }) {
     return MobileScannerState(
+      availableCameras: availableCameras ?? this.availableCameras,
       cameraDirection: cameraDirection ?? this.cameraDirection,
       error: error,
       isInitialized: isInitialized ?? this.isInitialized,