SetTimezone() and SetLocalRTC() updated the Context before writing
/etc/localtime or /etc/adjtime. If the write failed, D-Bus properties
showed the new value and a repeated request with the same value returned
success without retrying the file update.
Keep the write helpers Context-based, but save the old value before
installing the requested state. Restore the old Context value when the
backing file update fails.
Add TEST-45 coverage for failed alternate localtime and adjtime paths.
Follow-up for: f401e48c2d
dongshengyuan
844c44a0d4