make /dev/zero read-write

CTS test luni/src/test/java/libcore/java/nio/BufferTest.java function
testDevZeroMapRW() requires us to be able to open /dev/zero in read-write
mode. Allow it.

Change-Id: I2be266875b1d190188376fd84c0996039d3c1524

domain.te

@@ -64,7 +64,7 @@ allow domain device:file read;
 allow domain socket_device:dir r_dir_perms;
 allow domain owntty_device:chr_file rw_file_perms;
 allow domain null_device:chr_file rw_file_perms;
-allow domain zero_device:chr_file r_file_perms;
+allow domain zero_device:chr_file rw_file_perms;
 allow domain ashmem_device:chr_file rw_file_perms;
 allow domain binder_device:chr_file rw_file_perms;
 allow domain ptmx_device:chr_file rw_file_perms;