DO NOT MERGE ApfFilter: use elapsedRealTime for RA lifetime

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 305af8e9)

Change-Id: Idbde700025fecfecefb8162d66b94194a87829d5