Override the Exec resources, for root to run as the specified user.

Without this, attempting to run this as a non-root user exits with `Only root can execute commands as other users`.

_With_ this, any lowly CI user can install the Android SDK, by applying:
```
class { 'android':
  installdir => '/tmp/cached_sdk',
  version    => '23.0.2',
  user       => $id,
  group      => $id,
}
```