[Android] Decouple url loading and onReadyToRender callback

The url loading should not depend on the readiness of compositing
surface. Otherwise, it will cause 'deadlock' between loading url and
surface availability in some cases.

For example, to support splash screen in cordova, the XWalkView should
be initially invisible and waits for a signal from web page to become
visible. However, Android framework may not create the underlying Surface
for an invisibile View, which causes the url won't be loaded.

BUG=https://crosswalk-project.org/jira/browse/XWALK-1884
[Android] Improve the description of XWalkView

We should notice users to call onDestroy() method of XWalkView
explicitly, or it will cause the memory leak due to the native
stuff of XWalk is not released, such as the WebGL conext.

BUG=XWALK-1778
Merge pull request #2054 from xingnan/xwalkview-description

[Android] Improve the description of XWalkView
Merge pull request #2055 from hmin/xwalk-1884

[Android] Decouple url loading and onReadyToRender callback
[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