CB-7607 Add xxhdpi and xxxhdpi support to Android Cordova

https://issues.apache.org/jira/browse/CB-7607

Current Cordova template for Android does not have res sub-folders for xx and xxxhdpi plus due to current splash/icons support implementation it is possible to replace already existing drawables only(see var densities = this.deleteDefaultResource(...)). I don't think we should always keep new hi res images as part of default template due to the their big size so this commit improves splash/icons support implementation to make it possible to add drawables for the densities which are not present in template by default.

Example configuration for xxhdpi and xxxhdpi assets:
```
<icon src="res/android/xxhdpi.png" density="xxhdpi" />
<icon src="res/android/xxxhdpi.png" density="xxxhdpi" />

<splash src="res/android/land-xxhdpi.png" density="land-xxhdpi" />
<splash src="res/android/port-xxhdpi.png" density="port-xxhdpi" />

<splash src="res/android/land-xxxhdpi.png" density="land-xxxhdpi" />
<splash src="res/android/port-xxxhdpi.png" density="port-xxxhdpi" />
```