[Android] Make xwalk_core_library compilable on Windows

On windows, python is python.bat or python.exe.
Directly exec python will fail.
To detect which extension is used. Add condition
and "where" exec in precompile.xml to determine
the final extension string for python.

BUG=https://crosswalk-project.org/jira/browse/XWALK-335
Merge pull request #1111 from xingnan/webaudio

[Android] Enable Web Audio on x86
[Packaging] Correct typo in the README.
Bump version to 3.31.36.0
Bump version to 3.31.37.0
Merge pull request #1137 from rakuco/readme-typo

[Packaging] Correct typo in the README.
Bump version to 3.31.38.0
Merge pull request #1119 from wang16/core_pre_compile

[Android] Make xwalk_core_library compilable on Windows
[Android] Add back key support

1 The patch is to add the back key support for the runtime. The
expected behavior when putting on the back button is to go back to
the previous page, but now it results to exit the app.
2 Add a new goBack() interface into XWalkRuntimeView to handle this.
3 Hook the onKeyUp() method in all related Activities.

BUG=https://crosswalk-project.org/jira/browse/XWALK-83