Allow kernel to write to update_engine_data_file

This is needed to run update_engine unittests in cuttlefish. In the test,
the directory is mounted as R/W.

Denial:
avc: denied { write } for path="/data/misc/update_engine/tmp/a_img.NqUpaa" dev="dm-4" ino=3048 scontext=u:r:kernel:s0 tcontext=u:object_r:update_engine_data_file:s0 tclass=file permissive=0

strace:
mount("/dev/block/loop26", "/data/local/tmp/.org.chromium.Chromium.3s2KYE", "ext2", 0, "") = -1 EIO (I/O error)

Test: unittests pass
Change-Id: I4658eb60240bd725bac2aef30305747ffe50aeb6
diff --git a/public/kernel.te b/public/kernel.te
index 42fe2c4..35018e9 100644
--- a/public/kernel.te
+++ b/public/kernel.te
@@ -65,10 +65,10 @@
 allow kernel { app_data_file privapp_data_file }:file read;
 allow kernel asec_image_file:file read;
 
-# Allow reading loop device in update_engine_unittests. (b/28319454)
+# Allow mounting loop device in update_engine_unittests. (b/28319454)
 # and for LTP kernel tests (b/73220071)
 userdebug_or_eng(`
-  allow kernel update_engine_data_file:file read;
+  allow kernel update_engine_data_file:file { read write };
   allow kernel nativetest_data_file:file { read write };
 ')