Recommend different time source

We were recommending SystemClock#uptimeMillis() from Java code, but
millisecond resolution is a little coarse.  We don't have an
appropriate call in SystemClock, so use System.nanoTime() instead.

Bug 8733179

Change-Id: Ifb85a03eabf7ff168966adcb5ed5914bbf3884b2
1 file changed