Switching StatLogger to uptime

Uptime is a better estimate of time spent in cpu. Adding uptimeNanos to
SystemClock to get finer timestamps.

Test: Manual:
`adb shell dumpsys alarm` should show sensible information under 'Stats'
printed at the end.

Bug: 162791194
Change-Id: I4f1c9d41c53ec03fc414730d38fd7a2233bb0569