Return false when tetherOffloadRuleAdd doesn't really add items

This is needed for the follow-up CLs to check whether an item was
added when calling tetherOffloadRuleAdd. This is safe since no one
is actually checking the return value of this method.

Test: atest TetheringTests
Bug: 354619988
Change-Id: I28bafe3145bd2f62f9a7db9d0a300d2c7a148040

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

@@ -359,6 +359,7 @@ 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

@@ -140,6 +140,8 @@ public abstract class BpfCoordinatorShim {
 
     /**
      * Adds a tethering IPv4 offload rule to appropriate BPF map.
+     *
+     * @return true iff the map was modified, false if the key already exists or there was an error.
      */
     public abstract boolean tetherOffloadRuleAdd(boolean downstream, @NonNull Tether4Key key,
             @NonNull Tether4Value value);