[Android][Shared Core] Split xwalk core internal and xwalk core

1. Previous commit does:
  a. Move all xwalk core java classes include extension related classes
     to xwalk core internal.
  b. Rename all classes Embedding API exposes as "ClassInternal".
2. This commit does:
  a. Create a thin layer which is now new xwalk core, simply wraps core
     internal and overwrite the functions returning internal objects
     by creating and returning wrapped objects in xwalk core.
  b. Rename the inner class in core_internal which was exposed
     as Embedding API with "Internal" suffix.
  c. Create target core_internal_java and let core_java depends on it in gyp.
  d. Some necessary package name changing in tools and scripts.
  e. List all exposed API in xwalk core with comments to support
     API reference document generation.
[Android][Shared Core] Initialize org.xwalk.core.internal.R for xwalk core library

For core library usage, the internal resources ids are mapped to
org.xwalk.core.R at runtime dynamically to ensure the ids using
internally is correct.

Since now xwalk core uses org.xwalk.core.internal.R instead of
org.xwalk.core.R, the ids in org.xwalk.core.internal.R also
needs initialization before use at runtime.