Allow vendor_init to write to misc_block_device

Vendors may use this to write custom messages to their bootloader, and
as the bootloader is under vendor control, this makes sense to allow.

Bug: 77881566
Test: build
Merged-In: I78f80400e5f386cad1327a9209ee1afc8e334e56
Change-Id: I78f80400e5f386cad1327a9209ee1afc8e334e56
(cherry picked from commit db465285)

public/domain.te

@@ -597,6 +597,7 @@ neverallow {
   -init
   -uncrypt
   -update_engine
+  -vendor_init
   -vold
   -recovery
   -ueventd

public/vendor_init.te

@@ -146,6 +146,9 @@ allow vendor_init serialno_prop:file { getattr open read };
 # Vendor init can perform operations on trusted and security Extended Attributes
 allow vendor_init self:global_capability_class_set sys_admin;
 
+# Raw writes to misc block device
+allow vendor_init misc_block_device:blk_file w_file_perms;
+
 not_compatible_property(`
     set_prop(vendor_init, {
       property_type