[Android] Change the class modifier for the core library.

Set the classes defined in the core library with correct modifiers.
Previously lots of them are public which are not appropriate. Only
expose classes which will be for API. Some of them are still public
due to the usage of Cordova and Test Shell. Will do them.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1041
Merge pull request #1707 from yongsheng/embeddingapi

[Android] Change the class modifier for the core library.