[Android] Use script to generate core code which is a simple wrap of core internal

For each class of org.xwalk.core.internal which needs to be
exposed in Embedding API.
A bridge class and a wrapper class will be created for it.
The bridge class is a subclass of internal class and maintains
a wrapper object of this type.
A wrapper class is in org.xwalk.core and be exposed in Embedding
API, it maintains an instance of bridge object as well.
Wrapper and bridge talks to each other via pure reflection. So
they don't have to be in the same package in future.
I agree, i also considered to make all template declared as constant at the beginning to simplify this method. But i found it will be much more intuitive and easier to maintain if the content of the template is in the same screen of value dictionary.

I'll consider other means to shrink this method body.