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

core/java/android/net/http/SslCertificate.java

@@ -506,6 +506,6 @@ public class SslCertificate {
         if (certificateDate == null) {
             return "";
         }
-        return DateFormat.getDateFormat(context).format(certificateDate);
+        return DateFormat.getMediumDateFormat(context).format(certificateDate);
     }
 }