[Android] Replace postMessage() with createPromise() in messaging_api.js

postMessage() is not defined in this file. The correct function call is
createPromise().

BUG=XWALK-2877
Merge pull request #2581 from shawngao5/crosswalk-9_XWALK-2877

[Android] Replace postMessage() with createPromise() in messaging_api.js