[Runtime] Rename xwalk.app API to xwalk.app.runtime

In order to align with Chrome runtime API naming
(https://developer.chrome.com/apps/runtime.html), we changed xwalk.app.* APIs to
xwalk.app.runtime.*.
[Runtime] Move application runtime extension from extension thread to UI thread

After PR #1080(https://github.com/crosswalk-project/crosswalk/pull/1080) we can
run application extensions on UI thread now.

The internal application extension code used to run on extension thread while
most of the application service code runs on UI thread. This means now we don't
need to post task to UI thread and get its result back in extension thread back
and forth anymore. This will make the code more clean and straight forward.