Use proper digits in formatElapsedTime and format3339

Use getZeroDigit() instead of a hard-coded '0' for formatting times using
formatElapsedTime, so locales with different digits like Arabic and Persian
could display the elapsed time properly. This is visible in Settings' list
of running apps.

Also changed android.text.format.Time's format3339 method to always use ASCII
digits, irrespective of the locale.

Change-Id: I731c96c21b3712ec347d9526e4ec3fe884dec276