Prevent user ID reuse until after reboot.

We're still seeing rare cases where a device struggles to create a
new user, probably because of a subtle bug in the FUSE daemon.  To
work around this, only allow user IDs reuse after reboot.

Bug: 8302014
Change-Id: Id7f9fb539c6d6d1ff3d47d941af1d9e6b93eca03