Modifies statsd output for start and end times.

We include the start of when the last dump occurred and the current
timestamp. These timestamps are shared across all metrics, so
there's no advantage in duplicating these numbers across all metrics.

Also, we should use elapsed realtime instead of times based on wall
clock, which can jump around and go backwards.

Test: Test that statsd can still build and
adb shell cmd stats dump-report doesn't crash.
Change-Id: I819e5643cee75dfa3e78a58f94c9d61ededa78d7