The units for your extent can be found from REST service directory of your MapService.
And if you want to set the initExtent via Java than XML then you can define that in the constructor of your MapView object.
MapView
public MapView(android.content.Context context,
SpatialReference spatialreference,
Envelope extent)
The constructor is used if you are instantiating the MapView using Java code (see the class description for an example of this usage).
Parameters:
context - The Context the view is running in, through which it can access the current theme, resources, , and so on.
spatialreference - the spatial reference to initialize the map with.
extent - the initial map extent to display.