[Android] Refactor Java extension system

The commit is to decouple the extension from XWalkRuntimeViewProvider
and simplify the interfaces exposed by XWalkRuntimeViewProvider. Now it
is easy to use the extension system by creating a new
XWalkExtensionManager.

The backend implementation for XWalkExtension is abstracted as
XWalkExtensionBridge interface. Currently, one backend implementation
based on xwalk core is provided as default.

BUG=https://crosswalk-project.org/jira/browse/XWALK-148