Revert "Revert "Make init re-exec itself for its SELinux domain transition.""

This reverts commit 42173746.

It turns out that the kernel passes any unrecognized arguments on to init,
and (at least) N6 and N9 have such arguments. My lazy check of argc was
thus insufficient to recognize what stage of init we were in, so we'd
skip to stage 2 and not set up SELinux. And apparently you can get a
very long way with SELinux off... We'll fix that in a later change.

Bug: 19702273
Change-Id: I43b3fb722fed35dd217cb529cbcac9a29aff4e4b