init: Always create and restorecon /dev/dm-user.

Dynamically creating /dev/dm-user makes it difficult to set an inotify
watch for child devices to appear. It requires having an additional
watch on /dev, and more complicated sepolicy as a result.

Bug: 168554689
Test: full OTA with VABC enabled
Change-Id: I0cdef3d5a953a372446ff4e539a25a7b5314500d