Hide Android asset manager state from asset header

src/asset.h

@@ -1,11 +1,21 @@
 #pragma once
 
+#ifdef __ANDROID__
+struct AAsset;
+struct AAssetManager;
+#endif
+
 class Asset
 {
 public:
 #ifdef __ANDROID__
+    static void set_android_asset_manager(AAssetManager* asset_manager);
+
+private:
     static AAssetManager* m_am;
     AAsset* m_asset = nullptr;
+
+public:
 #endif
     static std::vector<std::string> list_files(std::string folder, const std::string& filter_regex);
     static bool exist(std::string path);