Fix faulty format specifier

Use the 0-23 format instead of 1-24.
Update stopped sleep session

Update the sleep session that is being stopped with the end time, just
in case it will get used to do something after.
Add Timespan class

Add a `Timespan' class that, for now, helps to create diffs of dates and
times. Currently only used for creating strings of these times.

Make the `SecondStepCounter' use it so that it stays consistent with
other places using the same thing.
Add method to get the latest sleep session
Show length of latest sleep session

Show the duration of the latest sleep session found in the database on
the main activity.