Tweak the layout algorithm for digital widget

Bug: 27282291

Changes in digital widget behavior:

- 12-hour time display no longer includes am/pm to match lock screen
- content is now centered within the bounds of the widget
- content is capped at the same size as the lock screen and scaled down
   to fit in the bounds of the widget

Change-Id: I32190222789524e326a699ac78fd51c9d80fbe9a