Tolerate negative stopwatch lap times

Bug: 32194582
Test: LapsAdapterTest.testFormatTimeIsNegativeMaxTime()

Negative stopwatch lap times should be impossible, but field
reports indicate otherwise. They are now formatted as 0.
Other changes will work to avoid this possibility, but if they do
occur, they are now handled gracefully.

Change-Id: I1d8b59ddf0634957cb8663a32c322400abc850f9