adb: Use Minijail for privilege dropping.

By using Minijail we avoid writing the same priv-dropping code over
and over again. This also enables future hardening opportunities.
We're already using Minijail for priv-dropping on Brillo.

Minijail is unit- and integration-tested on the
Chrome OS CI infrastructure (pulling from the same repo, see
https://wmatrix.googleplex.com/security?tests=security_Minijail0).
Once CI comes to Android, it will be continuously tested there as well.

Both 'adb root' and 'adb unroot' still work.

Bug: 26099611
Change-Id: I52c456ea272d27bd6fbc71200f4fdd928a592158