xdg-autostart-service: Use common boolean parser

xdg-autostart-service: Use common boolean parser

Technically the desktop entry specification says value should be the
string "true" or "false". Pragmatically every desktop has their own
parsing rules which are typically less strict on how to interpret other
values.

This caused some regressions downstream when we switched to the
xdg-autostart-generator where existing handmade files contained values
with "True" or "False".