Loading concurrent KML overlays does not work

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

**If you choose 'problem or bug report', please select OS:** (check one with "x")
- [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.

Start at src/@ionic-native/plugins/google-maps/index.ts around line 4918, then trace the KmlOverlay and Map entries shown in the stack trace. Reproduce the first and additional KML overlay creation with the listed Ionic Native Google Maps versions. Done means concurrent KML overlays are created without the Cannot redefine property: camera error.

Vom Indexierungsmodell aus dem Issue-Text verfasst.