优化设备状态的钥匙链读写。Review by 硬币。

@@ -399,16 +399,26 @@ static NSString * const kYohoBuyUsedDevice = @"com.yoho.buy.useddevice";
     if (yohoinfoKVPairs == nil) {
         yohoinfoKVPairs = [NSMutableDictionary dictionary];
     }
-    [yohoinfoKVPairs setObject:@"Y" forKey:kYohoBuyUsedDevice];
-    [YH_KeychainStore save:kYohoSecretInfo data:yohoinfoKVPairs];
+    
+    if (![yohoinfoKVPairs[kYohoBuyUsedDevice] isEqualToString:@"Y"]) {
+        [yohoinfoKVPairs setObject:@"Y" forKey:kYohoBuyUsedDevice];
+        [YH_KeychainStore save:kYohoSecretInfo data:yohoinfoKVPairs];
+    }
 }
 
 + (BOOL) isNewDevice
 {
+    static NSString * usedDevice = nil;
+    
+    if ([usedDevice isEqualToString:@"Y"]) {
+        return NO;
+    }
+    
     NSMutableDictionary *yohoinfoKVPairs = (NSMutableDictionary *)[YH_KeychainStore load:kYohoSecretInfo];
     if (yohoinfoKVPairs == nil || yohoinfoKVPairs[kYohoBuyUsedDevice] == nil) {
         return YES;
     } else {
+        usedDevice = yohoinfoKVPairs[kYohoBuyUsedDevice];
         return NO;
     }
 }