In Android, how to execute a service as a different user?

For example, is there any way to define a service to run as 'system'开发者_JAVA技巧 UID or GID?

Or any way to add your app to the 'system' group?

As noted in the comments on the question, this would create some security problems. There is no way to run an app or service as "system". (Barring bugs, of course.)