Use monotonic time for all session-lived intervals checks

Move all timestamp generation logic to HarvestSettings class,
and provide two methods for getting Real (monotonic) time and
Clock time, in seconds.

This way ALL the intervals check won't be affected by system
clock adjustments.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Start service from Activity

Do not wait until next re-start for starting the service, do
it also when the Activity is started for the first time.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Set proper debugging level in Log calls

This will avoid unnecessary messages spam when is running
in production.

Also fix some typos in the strings.

Signed-off-by: Martin Abente Lahaye <tch@sugarlabs.org>
Release 1.21

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