Fix elapsed time formatting.

We should extend DateUtils to provide this kind of elapsed time
formatting via icu4c, but for now let's at least fix our
arithmetic.

Bug: https://code.google.com/p/android/issues/detail?id=61479
Change-Id: I6c087dbb9fdbff0a21d9243d239126fee180454c
(cherry picked from commit 091f7f17610b7b5002bbc16be41fb36135548b70)
oops