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: If19011fc0c905948f2e42b975cfcc5f8672a95fb