Set idmap2 binary uid and gid after forking

The file permissions of the idmap2 binary are currently not set
correctly when the system forks and execs the idmap binary during
zygote. This chnages sets the uid and gid after forking to the same uid
and gid of the parent process.

Bug: 134897503
Test: device boots and generates idmap
Change-Id: Ic7fac49e5982f3c47713603b905c3a6be117a05b