Use libflutter_engine.so if inside asset bundle

Author: pieter-scholtz

include/flutter-pi.h

@@ -426,6 +426,7 @@ struct flutterpi {
 		char *app_elf_path;
 		void *app_elf_handle;
 		char *icu_data_path;
+		char *libflutter_engine_path;
 
 		FlutterLocale **locales;
 		size_t n_locales;