Enforce assumptions around metadata_block_device

Add a compile time assertion that only authorized SELinux domains are
allowed to touch the metadata_block_device. This domain may be wiped at
will, and we want to ensure that we're not inadvertently destroying
other people's data.

Test: policy compiles.
Change-Id: I9854b527c3d83e17f717d6cc8a1c6b50e0e373b6

public/domain.te

@@ -383,6 +383,12 @@ neverallow {
   -ueventd # Further restricted in ueventd.te
 } frp_block_device:blk_file rw_file_perms;
 
+# The metadata block device is set aside for device encryption and
+# verified boot metadata. It may be reset at will and should not
+# be used by other domains.
+neverallow { domain -init -recovery -vold } metadata_block_device:blk_file
+  { append link rename write open read ioctl lock };
+
 # No domain other than recovery and update_engine can write to system partition(s).
 neverallow { domain -recovery -update_engine } system_block_device:blk_file write;