adb/test_device.py fixes for win32 and no use of ANDROID_SERIAL

If ANDROID_SERIAL was not set, test_device.py was failing.

Use posixpath.join instead of os.path.join for make_random_device_files.

Change-Id: I24bfa43ba2a89a9a768f505fc0bba9d873082b2f
Signed-off-by: Spencer Low <CompareAndSwap@gmail.com>