Add /dev/kmsg_debug.

Add /dev/kmsg_debug on userdebug devices, to allow crash_dump to log
crashes to dmesg when logd isn't up yet (or is the one crashing).

(Originally commited in a015186f)
(cherry-pick of commit: 3458ec13)

Bug: 37916906
Bug: 36574794
Bug: 62101480
Test: Builds and boots.
Change-Id: I83aa392f49bb412d96534925fb02921a8f4731fa

prebuilts/api/26.0/26.0.ignore.cil

+;; new_objects - a collection of types that have been introduced that have no
+;;   analogue in older policy.  Thus, we do not need to map these types to
+;;   previous ones.  Add here to pass checkapi tests.
+(typeattribute new_objects)
+(typeattributeset new_objects (kmsg_debug_device))