[Android] Create documentation for XWalk Embedding API at compile time

Use javadoc to create API document based on Annotations
written in source code.
Need to use doclava to exclude the methods that are public
but not exposed as Embedding API.

The generated documents will be put in path
out/Debug(Release)/xwalk_core_library_docs/.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1856
Merge pull request #2043 from wang16/gen_docs

[Android] Create documentation for XWalk Embedding API at compile time