[CTT-6] Update TCP conntrack entry timeout while adding rules

Needed because a payload data packet may have gone through
non-offload path, before we added offload rules, and that this
may result in in-kernel conntrack state being in ESTABLISHED
but pending ACK (ie. UNACKED) state. But the in-kernel conntrack
might never see the ACK because we just added offload rules.
As such after adding the rules we need to force the timeout back
to the normal ESTABLISHED timeout of 5 days.

Issue: the timeout is set to unacknowledged 300s (countdwon to 298s)
$ adb shell cat /proc/net/nf_conntrack
ipv4     2 tcp      6 298 ESTABLISHED src=192.168.244.128
dst=140.112.8.116 sport=45694 dport=443 ..

Test: atest TetheringCoverageTests
Manual check:
$ adb shell cat /proc/net/nf_conntrack
ipv4     2 tcp      6 431988 ESTABLISHED src=192.168.40.162
dst=140.112.8.116 sport=40774 dport=443 ..

Bug: 190783768
Bug: 192804833

Change-Id: I8c34e85e26c9d976e5e2b85473db75ff46d8abd4

Tethering/apishim/31/com/android/networkstack/tethering/apishim/api31/BpfCoordinatorShimImpl.java

@@ -371,6 +371,7 @@ public class BpfCoordinatorShimImpl
         } catch (IllegalStateException e) {
             // Silent if the rule already exists. Note that the errno EEXIST was rethrown as
             // IllegalStateException. See BpfMap#insertEntry.
+            return false;
         }
         return true;
     }

Tethering/apishim/common/com/android/networkstack/tethering/apishim/common/BpfCoordinatorShim.java

@@ -141,6 +141,11 @@ public abstract class BpfCoordinatorShim {
 
     /**
      * Adds a tethering IPv4 offload rule to appropriate BPF map.
+     *
+     * @param downstream true if downstream, false if upstream.
+     * @param key the key to add.
+     * @param value the value to add.
+     * @return true iff the map was modified, false if the key exists or there was an error.
      */
     public abstract boolean tetherOffloadRuleAdd(boolean downstream, @NonNull Tether4Key key,
             @NonNull Tether4Value value);