[Fixup] split generator python into several scripts

Modulize and comments for maintanous in future.
[Fixup] Fix coding style according pylint result
[Android] Enable reflection layer generator

With the generator script introduced in previous commit,
we are now able to generate xwalk core classes automatically
from xwalk 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.

TODO: JavascriptInterface class needs to be placed in wrapper and
pass the class object to internal for recognition of javascript
interface.
[Android] Enable javascriptInterface under reflection framework

With reflection, core internal can't directly import
the JavascriptInterface class any more.
Add a static method to set the annotation class dynamically
when initializing xwalk.