Clean up Timer*Fragment classes

- Removed DeskClock.OnTapListener which just seemed to change the label
  text color in a pressed state (which can be done using a color state
  list if desired).
- Simplified timer_list_item layout to use a single TextView for the
  timer label.

Change-Id: I407f49b1852dc14546d3fb3f9ef53625320df07e