more locale-appropriate screenshot share text

The SimpleDateFormat version of this produces very poor results in
locales other than en_US. Trusting DateFormat to do its thing
produces more consistently-pleasing results across all locales.

(cherry-pick of e45e1f2efe69ba931efa88c6e33879fbc8d8c4c1.)

Change-Id: I7f1714446bb50285d851e8e313e7f32a0a822bf2