Localize the DatePicker date format

[colemancda]

The DatePicker's trailing button formatted its date with a hardcoded English month table ("Jan 1, 2000" everywhere). It now uses the device locale's medium date style via DateFormat.getDateInstance(MEDIUM), keeping the UTC timezone that matches the epoch-millis it's fed.

Verification

• Emulator (en-US, More Controls screen): the DatePicker shows "Jan 1, 2000" — unchanged for English.
• The formatter is genuinely locale-aware (ran the exact logic on the JVM across locales):
  • en-US → Jan 1, 2000
  • fr-FR → 1 janv. 2000
  • ja-JP → 2000/01/01
  • de-DE → 01.01.2000

Non-rooted emulator images don't allow a locale switch for a device screenshot, hence the JVM cross-locale check; the on-device en-US output matches the en-US column, confirming the same path.