kernel: allow write access to /data/misc/vold/virtual_disk

The kernel thread which manages this file really needs read/write access
to this file, not read-only. This was suspected in b/36626310 but
apparently something must have changed in the kernel surrounding
permission checking for kernel threads (still unknown)

Bug: 36626310
Bug: 117148019
Bug: 116841589
Test: policy compiles
Change-Id: I9c42541e2567a79b2d741eebf3ddf219f59478a9

public/kernel.te

@@ -79,7 +79,7 @@ allow kernel media_rw_data_file:dir create_dir_perms;
 allow kernel media_rw_data_file:file create_file_perms;
 
 # Access to /data/misc/vold/virtual_disk.
-allow kernel vold_data_file:file read;
+allow kernel vold_data_file:file { read write };
 
 # Allow the first-stage init (which is running in the kernel domain) to execute the
 # dynamic linker when it re-executes /init to switch into the second stage.