Fix age/gender i18n

Since the labels for the age-selection widget are taken from an
environment variable (no longer hard-coded) they are not included in
the strings for i18n. This patch adds default values to agepicker.py
so that they are included in the Sugar POT file.
Fix age/gender flake8 issues

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>