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

1. Previous commit does:
  a. Move all xwalk core java classes include extension related 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 xwalk core, simply wraps core
     internal and override the functions returns internal objects
     by creating and returnning wrapped objects in xwalk core.
  b. Rename the inner class which 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.
  d. Some necessary package name changing in tools and scripts.
  e. List all exposed API in xwalk core with comments to support
     API reference documente 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.