How do I disable keyframe interpolation on animations?

If you select all of the keyframes you can right click one and select Both Tangents -> Constant

I'm not 100% sure that will work for your imported animation, but its working well for my keyframed positions with sprite animations

If someone still with this question:

1. Select the wanted frames
   

2. Right Click

3. Select Flat (This will auto disable the option "AUTO" )

4. Don't mind for my english :)

I'm using Unity 5.2.0f3 however in my 4.1.2 version it's working too

I don't think there is a setting for that. You would have to set time of these animations to specific frames from script yourself.

this shit is awful, here is the script to disable it, thanks to one of the devs

     using UnityEditor;
     using UnityEditorInternal;
     using UnityEngine;
    
     static class TreeViewUtils
     {
         [MenuItem("TreeViewUtility/Toggle Animation")]
         static void ToggleAnimation()
         {
             const string prefKey = "TreeViewExpansionAnimation";
             bool newValue = !EditorPrefs.GetBool(prefKey, true);
             EditorPrefs.SetBool(prefKey, newValue);
             InternalEditorUtility.RequestScriptReload();
             Debug.Log("TreeView animation is now " + (newValue ? "enabled" : "disabled"));
         }
     }