sepolicy: add key search permission for init

To support insmod in init, key search permission is added so
that module key can be found for signature verification.

Address the following denial:
avc:  denied  { search } for  pid=1 comm="init" scontext=u:r:init:s0
 tcontext=u:r:kernel:s0 tclass=key permissive=0

Change-Id: I04d8289290fee923b09dee23dcf1333cc970d5cd