Determine default 12/24 hour time format setting from locale.

We allow the user to override the 12/24 hour format based on their
preference. Previously, the default was set to 24 hour format for all
users out of the box. Instead, initialize the default value from the
user's locale. It can always be changed later.

Change-Id: I9e4c5e9f2ebcb50321dd9d9438efc9b1a5ce6cdc