Use _exit for profile copy dexopt command

Previously we used exit(0), but this called global destructors and
could cause problems depending on the state when the forking
happened.

Using _exit avoids calling hte global destructors in the child
process.

Test: Delete packages.xml and flash to simulate first boot
Bug: 62597429
Change-Id: I3a6dcd5f05ca85e1488df154ec283c2ec842e59f