Show volume number in volume control ui

The volume can be controlled in 25 steps. Respectively, the ui shown
goes from 0 to 25.

An alternative would have been to display a percentage, but that would
mean that one volume button press would jump 4% up, which doesn't look
great.

Bug: 162302410
Test: flash system ui && click +/- volume controls
      Result: https://hsv.googleplex.com/5706562045739008
Change-Id: I295cf055937fae067f8ccc34eff656f42f85f072