Tweak AI isSub logic. (#11817)

Previously, it was based on getCanNotTarget(), but actually getCanEvade() is the better option since canNotTarget can be set on non-sub units. This also restores the sub purchase logic that was changed in caf296c.

I think this is a safer version of the change, since the above one may have had an effect where AI would start buying subs for defense, when it shouldn't.

Note: I verified all standard A&A maps and all the subs have canEvade set, so this should ensure the AI continues to behave as before on those maps.

game-app/game-core/src/main/java/games/strategy/triplea/ai/pro/data/ProPurchaseOption.java

@@ -92,7 +92,7 @@ public class ProPurchaseOption {
     transportCost = unitAttachment.getTransportCost() * quantity;
     carrierCost = unitAttachment.getCarrierCost() * quantity;
     isAir = unitAttachment.getIsAir();
-    isSub = !unitAttachment.getCanNotTarget().isEmpty();
+    isSub = unitAttachment.getCanEvade();
     isDestroyer = unitAttachment.getIsDestroyer();
     isTransport = unitAttachment.getTransportCapacity() > 0;
     isLandTransport = unitAttachment.isLandTransport();

game-app/game-core/src/main/java/games/strategy/triplea/ai/pro/data/ProPurchaseOptionMap.java

@@ -103,7 +103,9 @@ public ProPurchaseOptionMap(final GamePlayer player, final GameData data) {
         ProLogger.debug("Air: " + ppo);
       } else if (Matches.unitTypeIsSea().test(unitType)) {
         final ProPurchaseOption ppo = new ProPurchaseOption(rule, unitType, player, data);
-        seaDefenseOptions.add(ppo);
+        if (!ppo.isSub()) {
+          seaDefenseOptions.add(ppo);
+        }
         if (ppo.isTransport()) {
           seaTransportOptions.add(ppo);
         }