I believe from your question your network dataset has pre-existing travel modes and you want to switch between them to find different routing solutions.
If that is the case you can get the Travel Modes defined on the Network dataset using the route task's RouteTaskInfo. From this object you can get a list of TravelMode objects (getTravelModes).
Then on the RouteParameters object, that you pass into the RouteTask, you can use the setTravelMode to set the one you want to use for a particular solve .... as taken from the list of travel mode objects returned by the RouteTaskInfo as noted above.
See the help doc at
RouteParameters| arcgis-android and RouteTaskInfo| arcgis-android
Hope this helps.