camera: don't double-mirror front cam on Android

CameraX's Preview use case builds the SurfaceTexture transform
matrix with a horizontal mirror baked in for the front camera (so
the selfie preview reads naturally without consumer effort). Flutter
applies that matrix when sampling the texture. Adding our own
`Transform.flip(flipX: true)` on top double-mirrors — which on its
own would just un-mirror the selfie, but combined with CameraX's
counter-rotation when the device tilts, it makes the rotation appear
to *follow* the phone (i.e. tilt CW 90° → preview goes 90° more
CW). Removing the Flutter flip lets CameraX's matrix do the work on
its own.

iOS keeps the Flutter flip: AVCaptureConnection.isVideoMirrored on
the data output is `false` there (capture-time mirror would land
in the recorded MP4), so the preview-only mirror must live in the
widget tree.

lib/src/camera/camera_preview.dart

@@ -1,3 +1,5 @@
+import 'package:flutter/foundation.dart'
+    show defaultTargetPlatform, TargetPlatform;
 import 'package:flutter/widgets.dart';
 
 import 'camera.dart' show UxCameraController, UxCameraLens, UxCameraValue;
@@ -35,7 +37,17 @@ class UxCameraPreview extends StatelessWidget {
         if (turns != 0) {
           child = RotatedBox(quarterTurns: turns, child: child);
         }
-        if (value.description.lens == UxCameraLens.front) {
+        // iOS only: AVCaptureConnection.isVideoMirrored is `false` on
+        // the data output (capture stays raw), so Flutter mirrors the
+        // selfie preview here. On Android, CameraX's Preview use case
+        // builds a SurfaceTexture transform matrix that already
+        // mirrors the front cam — adding our own flip on top
+        // double-mirrors and, when the device tilts, makes CameraX's
+        // counter-rotation appear to double in the same direction as
+        // the phone tilt (because we're flipping the axis CameraX
+        // rotated around).
+        if (defaultTargetPlatform != TargetPlatform.android &&
+            value.description.lens == UxCameraLens.front) {
           child = Transform.flip(flipX: true, child: child);
         }
         return child;