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

1. This 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".
  c. 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.
  d. Rename the inner class in core_internal which was exposed
     as Embedding API with "Internal" suffix.
  e. Create target core_internal_java and let core_java depends on it in gyp.
  f. Some necessary package name changing in tools and scripts.
  g. 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.
Merge pull request #2002 from wang16/reflection_layer_push

[Android] [Shared Core] Move implementation of org.xwalk.core into org.xwalk.core.internal
[Android] Fix native filesystem API initialize crash for cordova

iWhen application doesn't declare "WRITE/READ_EXTERNAL_STORAGE" permission in
its Android.manifest, getExternalCacheDirectory() will still return null even
though sd card is mounted.For cordova usage, the permission is not added by
default.

BUG=XWALK-1952