[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 #1119 from wang16/core_pre_compile

[Android] Make xwalk_core_library compilable on Windows