vendor implementations need to be able to run modprobe as part of
init.rc scripts.  They cannot do so because of the strict neverallow
currently in place that disallows all coredomains (including init)
to execute vendor toybox.

Fix this by adding init to the exception list for the neverallow so
vendors can then run modprobe from .rc scripts and also add the rule to
allow init to transition to modprobe domain using vendor_toolbox.

Bug: b/38212864
Test: Boot sailfish

Change-Id: Ib839246954e9002859f3ba986094f206bfead137
Signed-off-by: Sandeep Patil <sspatil@google.com>