The misc_block_device partition is intended for the exclusive
use of the OTA system, and components related to the OTA system.
Disallow it's use by anyone else on user builds. On userdebug/eng
builds, allow any domain to use this, since this appears to be used
for testing purposes.

Bug: 26470876
Change-Id: I05d4ee025bb8a5e6a1a9237fefaa2b1c646e332c