Reduce memory churn for running stopwatches and timers

Bug: 28065873

String.format(...) was being called in a tight loop to format various
times (e.g. lap times, accumulated times, etc) when laps exist. This produces
more garbage for the VM to collect than is necessary. Formatting the
times directly into a reusable StringBuilder significantly reduces the
memory churn and removes intermittent GC pauses on older platforms
and low-memory devices.

Change-Id: Iad574cea7f827623bb4854458d43df76967d8c62