armory/blender/addon/armory_startup.py

import bpy
from addon_utils import enable
from bpy.app.handlers import persistent

@persistent
def handler(scene):
    preferences = bpy.context.preferences
    if not 'armory' in preferences.addons:
        enable('armory', default_set=True, persistent=True, handle_error=None)
        bpy.ops.wm.save_userpref()

def register():
    bpy.app.handlers.load_post.append(handler)

def unregister():
    pass

if __name__ == "__main__":
    register()