Set the fallback environment variables XDG_CONFIG_HOME and HOME to
make os.UserConfigDir and os.UserHomeDir report useful paths.
Signed-off-by: Elias Naur <mail@eliasnaur.com>
os.UserCacheDir can't work on Android because it doesn't have
access to the Java app context. Gio programs do have access, so set
up UserCacheDir's fallback, the XDG_CACHE_HOME environment variable.
Signed-off-by: Elias Naur <mail@eliasnaur.com>