Fix early init logging.

Remove the /dev/__kmsg__ workarounds (which can then be removed
from sepolicy), and fix confusion in the translation between
android-base logging and kernel logging priorities (in particular,
where 'notice' comes in the hierarchy).

Bug: http://b/30317429
Change-Id: I6eaf9919904b6b55bc402c20bf1a4ae269014bc7
Test: adb shell dmesg | grep init