add android integration & make iOS API consistent

add pubspec android plugin

update android CMakeLists

add filament android libs for linking

android/src/main/cpp/filament_api.h

@@ -0,0 +1,15 @@
+#include "FilamentViewer.hpp"
+#include <android/asset_manager.h>
+#include <android/asset_manager_jni.h>
+#include <android/native_window_jni.h>
+#include <android/log.h>
+
+void load_skybox(void* viewer, const char* skyboxPath, const char* iblPath);
+
+void* filament_viewer_new(
+    void* layer, 
+    const char* opaqueShaderPath, 
+    const char* fadeShaderPath, 
+    void* assetManager
+);
+}