[misc-xwalk] Move WebAudio sample to xwalk system tests

- App crashes after some time because of Audio Decoder
- See https://crosswalk-project.org/jira/browse/XWALK-1993

Impacted TCs num(approved): New 0, Update 1, Delete 0
Unit test Platform: Android
Test result summary: Pass 0, Fail 1, Blocked 0
@zqzhang If purpose is too long, On-screen display incomplete.