[split system] Tentatively support running DO on meat user

- setDeviceOwner() now takes a user ID.  (We can infer it from Binder, but
we still need it for the dpm command.)

- Change broadcast target UID for DO to the DO user

- Start the DO user on boot complete.
TODO Investigate whether this is actually the good timing.

TODO Prevent the DO user from being killed

Bug 23827706

Change-Id: I227dbd444f1f4e94d98c317489d151554fe79d91