Add policy for 'blank_screen'.

This util allows init to turn off the screen
without any binder dependencies.

Bug: 70846424
Test: manual + init use
Change-Id: I4f41a966d6398e959ea6baf36c2cfe6fcebc00de
diff --git a/private/file_contexts b/private/file_contexts
index 8dfec78..6a94951 100644
--- a/private/file_contexts
+++ b/private/file_contexts
@@ -179,6 +179,7 @@
 #
 /system(/.*)?		u:object_r:system_file:s0
 /system/bin/atrace	u:object_r:atrace_exec:s0
+/system/bin/blank_screen	u:object_r:blank_screen_exec:s0
 /system/bin/e2fsdroid		u:object_r:e2fs_exec:s0
 /system/bin/mke2fs		u:object_r:e2fs_exec:s0
 /system/bin/e2fsck	--	u:object_r:fsck_exec:s0