SslCertificate should provide Date interface

While working on out openssl code, I found a Y2k bug that the dates
from invalidate certificates could be shown as 1909 instead of 2009.
The reason was because SslCertificate/BrowserActivity passed the
values around as Strings even though the started as Dates (from
X509Certificate) and were converted backed to Dates before
presentation by BrowserActivity's reformatCertificateDate.

SslCertificate now maintains date fields internally as Date objects
without converting them to Strings. The constructor and String
accessors, which are now @deprecated, now specify the format as an ISO
8601 date string which uses 4 digit years.

BrowserActivity now reformatCertificateDate is now simply
formatCertificateDate and no longer has to convert from String to Date
and back to String to get proper Locale formatting.

CTS SslCertificateTest also updated.