This patch replaces System.currentTimeMillis() with
SystemClock.elapsedRealTime() to make RA lifetime computation more
resilient to various external events inducing jumps in
currentTimeMillis().
Test: ApfTest passes.
(cherry picked from commit 305af8e98a)
Change-Id: Idbde700025fecfecefb8162d66b94194a87829d5