run-as: Use Minijail for privilege dropping.

Arguably, we don't need a ScopedMinijail for a program that only execs,
but I'd rather keep the code consistent and have all uses of Minijail
be good examples.

Bug: 30156807

Change-Id: I08a968835e0f3e2afcd5e7736626edbed658cde2
2 files changed