Move DateFormat.zeroPad() back to upstream.

We can only safely use TextUtils.formatSimple() when formatting
strings that don't appear in UI, since locales can use different
glyphs for representing numerals.

Bug: 174576537
Test: manual
Exempt-From-Owner-Approval: revert
Change-Id: Iafba1ccde239e3053b2a462c3d0c248795fbf5fc