FDroidPopen must have a locale to support UTF-8 filenames

`fdroid update` should be able to handle any valid filename (hopefully
aapt doesn't barf on them).  To handle that, the environment where the
shell commands are run in needs to have a UTF-8 locale set. If LANG is
not set, things default to ASCII and UTF-8 filenames fail.

This also renames test APK with lots of Unicode chars as a test case.

closes #167