fastbootd: allow to fetch vendor_boot partition.

vendor_boot is labeled as boot_block_device. With
fastboot fetch command, fastbootd needs to read
the vendor_boot device and return it to the host.

Test: pass
Bug: 173654501
Change-Id: I197e39c9e7572dc9a714f36637c02ee9ead2e5f3
diff --git a/public/fastbootd.te b/public/fastbootd.te
index fb3e953..9614545 100644
--- a/public/fastbootd.te
+++ b/public/fastbootd.te
@@ -98,6 +98,8 @@
     }:{ file lnk_file } unlink;
     allow fastbootd tmpfs:dir rw_dir_perms;
     allow fastbootd labeledfs:filesystem { mount unmount };
+    # Fetch vendor_boot partition
+    allow fastbootd boot_block_device:blk_file r_file_perms;
   ')
 
   # Allow using libfiemap/gsid directly (no binder in recovery).