[misc][XWALK-1585] Fix case bug for webaudio

- Change the way to create AudioContext object

Impacted TCs num(approved): New 0, Update 1, Delete 0
Unit test Platform: [Andriod]
Andriod test result summary: Pass 1, Fail 0, Blocked 0
Merge pull request #325 from JianfengXu/xujf/misc/webaudio

[misc][XWALK-1585] Fix case bug for webaudio