Fix DateUtils.formatElapsedTime.

More reuse of StringBuilders, less broken home-grown formatting code.

Long-term, we should hand this over to icu4c, but they're not ready yet.

Bug: http://code.google.com/p/android/issues/detail?id=41401
Bug: 7736688
Change-Id: Ib3c1e1aad05827df646aa18645cce19dffb7551f