kernel: make kernel an mlstrustedsubject

Addresses post-review comment in
https://android-review.googlesource.com/130620

Change-Id: I427ba99d63724eb526d41da47b95cc0ae038acdd

kernel.te

 # Life begins with the kernel.
-type kernel, domain;
+type kernel, domain, mlstrustedsubject;
 
 allow kernel self:capability sys_nice;