Hide Android asset manager state from asset header

src/asset.cpp

@@ -9,9 +9,15 @@
 #endif
 
 #ifdef __ANDROID__
+#include <android/asset_manager.h>
 #include <dirent.h>
 AAssetManager* Asset::m_am;
 bool android_create_dir(const std::string& path);
+
+void Asset::set_android_asset_manager(AAssetManager* asset_manager)
+{
+    m_am = asset_manager;
+}
 #endif
 
 bool Asset::delete_file(const std::string& path)