Loading concurrent KML overlays does not work

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- [x] any problem or bug report
- [ ] feature request

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- [x] **Android**
- [ ] **iOS**
- [x] **Browser**

**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.