[Android] Split crosswalk into core layer and runtime layer.

Originally the two layers are compiled into one Java package. It needs to
split them into two different directories for building because core will
be published as an Android library without runtime layer code.
Besides that, there are also 2 major changes:
1. Move Java extension mechanism and extensions inside core layer.
2. Change the 'runtimelib' to 'runtime_lib'.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1041