Android: prevent misc obfuscation for gltfio

Fixes #5944

android/filament-android/src/main/java/com/google/android/filament/Material.java

@@ -36,6 +36,7 @@ import java.util.Set;
  *
  * @see <a href="https://google.github.io/filament/Materials.html">Filament Materials Guide</a>
  */
+@UsedByNative("AssetLoader.cpp")
 public class Material {
     static final class EnumCache {
         private EnumCache() { }

android/filament-android/src/main/java/com/google/android/filament/MaterialInstance.java

@@ -20,6 +20,9 @@ import androidx.annotation.IntRange;
 import androidx.annotation.NonNull;
 import androidx.annotation.Size;
 
+import com.google.android.filament.proguard.UsedByNative;
+
+@UsedByNative("AssetLoader.cpp")
 public class MaterialInstance {
     private Material mMaterial;
     private String mName;

android/gltfio-android/src/main/java/com/google/android/filament/gltfio/MaterialProvider.java

@@ -24,12 +24,16 @@ import androidx.annotation.NonNull;
 import androidx.annotation.Nullable;
 import androidx.annotation.Size;
 
+import com.google.android.filament.proguard.UsedByNative;
+
+@UsedByNative("AssetLoader.cpp")
 public interface MaterialProvider {
 
     /**
      * MaterialKey specifies the requirements for a requested glTF material.
      * The provider creates Filament materials that fulfill these requirements.
      */
+    @UsedByNative("MaterialKey.cpp")
     public static class MaterialKey {
         public boolean doubleSided;
         public boolean unlit;