Consistent creation/destruction of user data.

Preparing and destroying users currently needs to be split across
installd, system_server, and vold, since no single party has all the
required SELinux permissions.

When preparing user directories on a storage device, always enforce
the serial number and destroy data if we run into a mismatch.  When
deleting a user, write the updated user list first before we start
destroying data.  Also start reconciling users on internal storage
at boot, so we can recover from stale data left behind from partially
destroyed users.

Check both CE and DE user directories when reconciling user storage
on a newly mounted storage device.

Bug: 27896918
Change-Id: I4536c82b0196e2720628c4f73fccb742c233350b