[Android] Fixed make_apk.py does not work on windows

This issue caused by Python deal with subprocess.Popen and subprocess.call is different on Windows and Linux. On Windows, PATH is only considered when shell=True is also passed.

Another issue caused make_apk.py failed is that when run follow command on Windows, 
"ant release -f Test\build.xml -quiet -Dkey.store="C:..\crosswalk-9.
37.198.0\xwalk-debug.keystore" -Dkey.alias="xwalkdebugkey" -Dkey.store.password="xwalkdebug" -Dkey.alias.password="xwalkdebug"
The result is error: "exited with non-zero exit code 1" So change the ant target to dubug.

BUG=https://crosswalk-project.org/jira/browse/XWALK-2377
There is no need to call it `shell_` instead of `shell`. And you can just use `shell = sys.platform == 'win32'`.