Use SystemClock.elapsedRealtime() to record WebView startup timestamps.

ag/13435848 introduces timestamp collections based on
System.currentTimeMillis but elapsedRealtime is a better
choice for a interval timing.

Bug: 180489435
Test: n/a
Change-Id: If7b29cbe90e8bba76bccb42a5bc570c08b75c9e7