You can get a standalone APK, i.e. without an extension OBB, by using the “Package .pak in APK” project packaging option. Obviously you will need to also enable the PAK file. As for source for that packaging it’s mostly in AndroidPlatform.Automation.cs. There’s also some code for dealing with the OBB packaging in UEDeployAndroid.cs.
Note: There are significant changes to the OBB and packaging in the next release and following.