Implement a full 24h mode in Clock activity

[llaske]

A new feature #903 of Clock activity added a 24h mode to display 24h value for hours.

The idea is to improve this mode to:

• Use this settings for digits too
• Change background of the clock for PM hours (may be from white to black)
• Allow to pass to AM and PM when rotating the clock arrows
• Ask questions for AM and PM in play mode

[saurabhhere]

For PM hours, do we need something like this?

[llaske]

That's the idea.
Did you change the hours colors? It seems brighter than the AM version. I think it's better if colors are the same in AM/PM mode.

[saurabhhere]

No, I haven't updated the hours colors. Those are the same as before. Please have a look at #923

[llaske]

Fixed in #923