Introduce world city widget

Bug: 27282291
Bug: 27693716
Bug: 24126517

This CL includes:
- full implementation of city widget
- event logging for creation/deletion of widgets
- tapping all widgets now opens the app in the prior state
  (not the clock tab)

Change-Id: Ia43ec0ebf2f02aba6b7e277e5feca3e8afa91e64