Enable SELinux on vold

This change enables SELinux security enforcement on vold.

For the vold.te file ONLY, this change is conceptually a revert of
77d4731e and
50e37b93, with the following
additional changes:

1) Removal of "allow vold proc:file write;" and
"allow vold self:capability { sys_boot };". As of system/vold
change adfba3626e76c1931649634275d241b226cd1b9a, vold no longer
performs it's own reboots, so these capabilities are no longer
needed.

2) Addition of the powerctl property, which vold contacts to
tell init to reboot.

3) Removal of "allow vold kernel:system module_request;". As of
CTS commit f2cfdf5c057140d9442fcfeb4e4a648e8258b659, Android
devices no longer ship with loadable modules, hence we don't
require this rule.

4) Removal of "fsetid" from "self:capability". Any setuid / setgid
bits SHOULD be cleared if vold is able to change the permissions
of files. IMHO, it was a mistake to ever include this capability in
the first place.

Testing: As much as possible, I've tested filesystem related
functionality, including factory reset and device encryption.
I wasn't able to test fstrim functionality, which is a fairly
new feature.  I didn't see any policy denials in dmesg. It's quite
possible I've missed something. If we experience problems, I
happy to roll back this change.

Bug: 9629920
Change-Id: I683afa0dffe9f28952287bfdb7ee4e0423c2e97a