xxhdpi/xxxhdpi and 9-patch splash image support for Android

This combines #89 and #87 so that they won't conflict with each other.
/cc @sgrebnov 

From #89:
> 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" />
> ```

From #87:
> https://issues.apache.org/jira/browse/CB-7598
>
> If a splashscreen image for Android is specified in config.xml and ends with .9.png, it is a 9-patch image
> and should retain that extension when copied into the platforms/android directory.
>
> Currently the file will be renamed to screen.png when it should be screen.9.png.