feat(android): enable loading of assets outside of the content web asset directory

[carlpoole]

Adds an override option for the RouteProcessor to allow Portals to request a route outside of the assetPath location. EG: a Portal at /assets/mywebapp/index.html wants to load an image at /assets/sharedimages/fish.jpg

Example usage in Portals:

// If AssetMap contains this virtual route, reroute to shared assets location
assetLoop@ for ((mapName, assetMap) in assetMaps) {
    if (path != null) {
        if(path.startsWith(assetMap.virtualPath)) {
            val assetMapObj = assetMaps[mapName]
            if (assetMapObj != null) {
                var trimmedPath = path.replace(assetMap.virtualPath,assetMapObj.getAssetPath())
                if (trimmedPath.startsWith("/")) {
                    trimmedPath = trimmedPath.drop(1)
                }

                processedRoute.path = trimmedPath
                processedRoute.isAsset = true
                processedRoute.isIgnoreAssetPath = true
                break@assetLoop
            }
        }
    }
}