Display longer date string in certificate information

Previously the date string used for "issued on" and "expires on" fields
are in short format (e.g. 2/3/17), which is ambiguous. In this CL we
use the date format of medium length (e.g. 2 Mar 2017) which also
adjusts automatically to the system locale.

Bug: 34428233
Test: manual, by opening Settings > Security > Certificates > any
      certificate

Change-Id: I5eff133d5c621e3e9969c7cf22ec757bd2b48dc4