android: allow modelviewer to reset (#9813)

- Add reset functionality to ModelViewer
 - We use this reset in render-validation to not have to reload
   the scene in runs that use the same model.
 - Fix a bug where the camera manipulator is disabling setting
   the camera via view config.

android/filament-utils-android/src/main/java/com/google/android/filament/utils/ModelViewer.kt

@@ -266,6 +266,35 @@ class ModelViewer(
         }
     }
 
+    /**
+     * Resets the model's transform, animation, and camera state to defaults.
+     * Call this when reusing the same model across multiple tests.
+     */
+    fun resetToDefaultState() {
+        // 1. Reset Camera parameters
+        cameraFocalLength = 28f
+        cameraNear = kNearPlane
+        cameraFar = kFarPlane
+        updateCameraProjection()
+
+        // 2. Reset the manipulator's look-at vectors to initial state
+        cameraManipulator?.let { cm ->
+            cm.jumpToBookmark(cm.homeBookmark)
+        }
+
+        // 3. Reset Animations
+        animator?.let {
+            if (it.animationCount > 0) {
+                it.applyAnimation(0, 0.0f)
+            }
+            it.updateBoneMatrices()
+        }
+
+        // 4. Re-apply the unit cube transform to clear custom scaling/translation
+        clearRootTransform()
+        transformToUnitCube()
+    }
+
     /**
      * Frees all entities associated with the most recently-loaded model.
      */

android/samples/sample-render-validation/src/main/java/com/google/android/filament/validation/MainActivity.kt

@@ -175,7 +175,7 @@ class MainActivity : Activity(), ValidationRunner.Callback {
         window.addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON)
 
         choreographer = Choreographer.getInstance()
-        modelViewer = ModelViewer(surfaceView)
+        modelViewer = ModelViewer(surfaceView=surfaceView, manipulator=null)
         inputManager = ValidationInputManager(this)
 
         // Initialize IBL

android/samples/sample-render-validation/src/main/java/com/google/android/filament/validation/ValidationRunner.kt

@@ -81,6 +81,17 @@ class ValidationRunner(
     }
 
     private fun startModel(modelName: String) {
+        if (currentModelName == modelName) {
+            Log.i("ValidationRunner", "Reusing model $modelName")
+            callback?.onStatusChanged("Reusing $modelName for ${currentTestConfig?.name}")
+
+            modelViewer.resetToDefaultState()
+
+            frameCounter = 0
+            currentState = State.WARMUP
+            return
+        }
+
         currentModelName = modelName
         val modelPath = config.models[modelName]
         if (modelPath == null) {