Oh it's pretty simple. Let's say I put my batch file at the root of my project. Then I create a directory tree similar to this:
mybatchfile.bat
-- icons
---- AndroidRes
-------- mipmap-anydpi-v26
-------- mipmap-hdpi
-------- mipmap-mdpi
-------- mipmap-xdpi
-------- mipmap-xxdpi
-------- mipmap-xxxdpi
-------- values
If my AIR SDK is stored in D:\AIRSDK, then I simply put this line in my batch file:
xcopy icons\AndroidRes D:\AIRSDK\lib\android\lib\resources\app_entry\res /S /Y
It copies the content of AndroidRes into the corresponding folder of the AIR SDK.