[general] moving ios and android project under ./platform folder

platform/android/MapboxGLAndroidSDK/src/cpp/main.cpp

@@ -0,0 +1,9 @@
+#include "jni.hpp"
+#include "jni_native.hpp"
+#include <jni/jni.hpp>
+
+extern "C" JNIEXPORT jint JNI_OnLoad(JavaVM *vm, void *) {
+    assert(vm != nullptr);
+    mbgl::android::registerNatives(vm);
+    return JNI_VERSION_1_6;
+}