Loading concurrent KML overlays does not work

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**cordova information:** (run `$> cordova plugin list`)

```
cordova-plugin-advanced-http 3.1.0 "Advanced HTTP plugin"
cordova-plugin-android-permissions 1.1.0 "Permissions"
cordova-plugin-app-version 0.1.12 "AppVersion"
cordova-plugin-bluetooth-serial 0.4.7 "Bluetooth Serial"
cordova-plugin-device 2.0.3 "Device"
cordova-plugin-file 6.0.2 "File"
cordova-plugin-filepath 1.5.8 "cordova-plugin-filepath"
cordova-plugin-fullscreen 1.3.0 "cordova-plugin-fullscreen"
cordova-plugin-geolocation 4.0.2 "Geolocation"
cordova-plugin-googlemaps 2.7.1 "cordova-plugin-googlemaps"
cordova-plugin-insomnia 4.3.0 "Insomnia (prevent screen sleep)"
cordova-plugin-ionic-webview 4.2.1 "cordova-plugin-ionic-webview"
cordova-plugin-network-information 2.0.2 "Network Information"
cordova-plugin-request-location-accuracy 2.3.0 "Request Location Accuracy"
cordova-plugin-screen-orientation 3.0.2 "Screen Orientation"
cordova-plugin-splashscreen 5.0.4 "Splashscreen"
cordova-plugin-statusbar 2.4.3 "StatusBar"
cordova-plugin-whitelist 1.3.4 "Whitelist"
cordova-plugin-wifiwizard2 3.1.1 "WifiWizard2"
es6-promise-plugin 4.1.0 "Promise"
```

**If you use `@ionic-native/google-maps`, please tell the package.json (only `@ionic-native/core` and `@ionic-native/google-maps` are fine mostly)**

```
"@ionic-native/core": "^5.29.0",
"@ionic-native/google-maps": "^5.5.0",
```

**Current behavior:**
When you create a first KML overlay, everything is fine. However, once you create an additional KML overlay the following error is triggered:

```
Error: Uncaught (in promise): TypeError: Cannot redefine property: camera
TypeError: Cannot redefine property: camera
at Function.defineProperty ()
at new KmlOverlay (index.js:1863)
at Map. (index.js:1598)
at KmlLoader. (Map.js:1050)
at BaseArrayClass. (KmlLoader.js:87)
```

From what I can see it seems like the problem is the following:

https://github.com/ionic-team/ionic-native-google-maps/blob/c0ff40e6fdf2bc156982899dde361352531db52d/src/%40ionic-native/plugins/google-maps/index.ts#L4918

The self variable does not seem to be defined here anywhere?

**Expected behavior:**
Any concurrent KML overlays should be created without any problem just like the first one.

Bắt đầu tại src/@ionic-native/plugins/google-maps/index.ts, khoảng dòng 4918, sau đó lần theo các mục KmlOverlay và Map được hiển thị trong stack trace. Tái hiện việc tạo overlay KML đầu tiên và các overlay KML bổ sung bằng các phiên bản Ionic Native Google Maps được liệt kê. Được xem là hoàn thành khi có thể tạo đồng thời các overlay KML mà không gặp lỗi Cannot redefine property: camera.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.