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 305af8e98a)

Change-Id: Idbde700025fecfecefb8162d66b94194a87829d5

services/net/java/android/net/apf/ApfFilter.java

@@ -285,10 +285,9 @@ public class ApfFilter {
         mReceiveThread.start();
     }
 
-    // Returns seconds since Unix Epoch.
+    // Returns seconds since device boot.
-    // TODO: use SystemClock.elapsedRealtime() instead
     private static long curTime() {
-        return System.currentTimeMillis() / DateUtils.SECOND_IN_MILLIS;
+        return SystemClock.elapsedRealtime() / DateUtils.SECOND_IN_MILLIS;
     }
 
     public static class InvalidRaException extends Exception {